i have tried many things but nothing works. I'm NOT using a virtual machine.

at last i have tried doing:

sudo apt remove realtek-rtl88xxau-dkms
sudo apt autoremove

sudo apt install realtek-rtl88xxau-dkms
and output was:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
dkms
Suggested packages:
python3-apport
The following NEW packages will be installed:
dkms realtek-rtl88xxau-dkms
0 upgraded, 2 newly installed, 0 to remove and 1639 not upgraded.
Need to get 1,752 kB/1,827 kB of archives.
After this operation, 18.5 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://ftp.free.fr/pub/kali kali-rolling/contrib amd64 realtek-rtl88xxau-dkms all 5.1.5~20171206-0kali1 [1,752 kB]
Fetched 1,752 kB in 4s (400 kB/s)
Selecting previously unselected package dkms.
(Reading database ... 323625 files and directories currently installed.)
Preparing to unpack .../archives/dkms_2.3-3_all.deb ...
Unpacking dkms (2.3-3) ...
Selecting previously unselected package realtek-rtl88xxau-dkms.
Preparing to unpack .../realtek-rtl88xxau-dkms_5.1.5~20171206-0kali1_all.deb ...
Unpacking realtek-rtl88xxau-dkms (5.1.5~20171206-0kali1) ...
Setting up dkms (2.3-3) ...
Processing triggers for man-db (2.7.6.1-2) ...
Setting up realtek-rtl88xxau-dkms (5.1.5~20171206-0kali1) ...
Loading new realtek-rtl88xxau-5.1.5~20171206 DKMS files...
Building for 4.9.0-kali3-amd64
Building initial module for 4.9.0-kali3-amd64
Error! Bad return status for module build on kernel: 4.9.0-kali3-amd64 (x86_64)
Consult /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/make.log for more information.

in this make.log file

DKMS make.log for realtek-rtl88xxau-5.1.5~20171206 for kernel 4.9.0-kali3-amd64 (x86_64)
Thu Dec 14 21:43:51 CET 2017
make ARCH=x86_64 CROSS_COMPILE= -C /lib/modules/4.9.0-kali3-amd64/build M=/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build modules
make[1]: Entering directory '/usr/src/linux-headers-4.14.0-kali1-amd64'
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_cmd.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_security.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_debug.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_io.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_ioctl_query.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_ioctl_set.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_ieee80211.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_mlme.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_mlme_ext.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_mi.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_wlan_util.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_vht.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_pwrctrl.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_rf.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_recv.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_sta_mgt.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_ap.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_xmit.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_p2p.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_tdls.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_br_ext.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_iol.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_sreset.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_btcoex.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_beamforming.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/rtw_odm.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/core/efuse/rtw_efuse.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/osdep_service.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/os_intfs.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/usb_intf.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/usb_ops_linux.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_linux.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/xmit_linux.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/mlme_linux.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/recv_linux.o
CC [M] /var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.o
/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.c: In function ‘rtw_spt_band_alloc’:
/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.c:238:12: error: ‘struct ieee80211_supported_band’ has no member named ‘channels’; did you mean ‘n_channels’?
spt_band->channels = (struct ieee80211_channel *)(((u8 *)spt_band) + sizeof(struct ieee80211_supported_band));
^~~~~~~~
n_channels
/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.c:239:66: error: ‘struct ieee80211_supported_band’ has no member named ‘channels’; did you mean ‘n_channels’?
spt_band->bitrates = (struct ieee80211_rate *)(((u8 *)spt_band->channels) + sizeof(struct ieee80211_channel) * n_channels);
^~~~~~~~
n_channels
/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.c:245:34: error: ‘struct ieee80211_supported_band’ has no member named ‘channels’; did you mean ‘n_channels’?
rtw_2g_channels_init(spt_band->channels);
^~~~~~~~
n_channels
/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.c:248:34: error: ‘struct ieee80211_supported_band’ has no member named ‘channels’; did you mean ‘n_channels’?
rtw_5g_channels_init(spt_band->channels);
^~~~~~~~
n_channels
/usr/src/linux-headers-4.14.0-kali1-common/scripts/Makefile.build:319: recipe for target '/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.o' failed
make[4]: *** [/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build/os_dep/linux/ioctl_cfg80211.o] Error 1
/usr/src/linux-headers-4.14.0-kali1-common/Makefile:1520: recipe for target '_module_/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build' failed
make[3]: *** [_module_/var/lib/dkms/realtek-rtl88xxau/5.1.5~20171206/build] Error 2
Makefile:146: recipe for target 'sub-make' failed
make[2]: *** [sub-make] Error 2
Makefile:8: recipe for target 'all' failed
make[1]: *** [all] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-4.14.0-kali1-amd64'
Makefile:1857: recipe for target 'modules' failed
make: *** [modules] Error 2

Why do not you build your own kernel ?>

Configuration hint
your current kernel config

Please read this, then read this.

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