I am running Ubuntu under Virtual Box 6.1.22. I am getting "Low Disk Space on "Filesystem root"".

user1@Ubuntu-VB:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.2 LTS
Release: 20.04
Codename: focal


user1@Ubuntu-VB:~$ df -h
Filesystem Size Used Avail Use% Mounted on
udev 1.9G 0 1.9G 0% /dev
tmpfs 394M 1.5M 393M 1% /run
/dev/sda5 9.3G 8.4G 475M 95% /
tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 2.0G 0 2.0G 0% /sys/fs/cgroup
/dev/loop1 56M 56M 0 100% /snap/core18/2074
/dev/loop0 56M 56M 0 100% /snap/core18/1997
/dev/loop4 66M 66M 0 100% /snap/gtk-common-themes/1515
/dev/loop3 65M 65M 0 100% /snap/gtk-common-themes/1514
/dev/loop2 132M 132M 0 100% /snap/docker/796
/dev/loop6 52M 52M 0 100% /snap/snap-store/518
/dev/loop5 219M 219M 0 100% /snap/gnome-3-34-1804/66
/dev/loop8 51M 51M 0 100% /snap/snap-store/547
/dev/loop7 219M 219M 0 100% /snap/gnome-3-34-1804/72
/dev/loop9 33M 33M 0 100% /snap/snapd/11402
/dev/loop10 33M 33M 0 100% /snap/snapd/12398
/dev/sda1 511M 4.0K 511M 1% /boot/efi
VM_Share 222G 167G 55G 76% /media/sf_VM_Share
tmpfs 394M 28K 394M 1% /run/user/1000
/dev/sr0 59M 59M 0 100% /media/user1/VBox_GAs_6.1.22

df -h output shows that / is 95%.

My original VM setup for storage (ubuntu.vdi) was 10G disk space and space is dynmaically allocated. The VM setting shows Virtual size of 10G and actual size of 9.5G

I used "VBoxManage modifyhd" command to increase the disk space from 10G to 30G and restart Ubuntu. Now the VM setting shows Virtual size of 30G and actual size of 8.99G.
I did some searching. I think I need to use repartition software to increase /dev/sda5. The search results I read was using fixed allocation for VM disk. I am not sure if Dynamic allocation works the same way. I need some help on if I should convert my disk to fix allocation first or I can stay with dynamic allocation and space will be acquired when I expand /dev/sda5.

/dev/sda5 9.3G 8.4G 475M 95% /

That is almost full.

Boot to partition software in VM. Gparted or whatever Ubuntu ships with will be fine to see if you do have any free space in the Virtual Hard Drive. Don't manage root while booted, you need to boot to some other live media.

In addition, the added space is at the end of the virtual drive but how to add it to /dev/sda5 depends how the drive is partitioned i.e MBR versus GPT and where is swap. Post the output of the command:

lsblk

Once the added space is adjacent i.e to the "right" of sda5 you can simply resize your / filesystem.