Hello I just stuck with very naive error ..!!!
Here it is

root@ubuntu:/home/ramzan# nios2-linux-uclibc-g (PRESSING TAB)
nios2-linux-uclibc-g++
nios2-linux-uclibc-gcc
nios2-linux-uclibc-gcc-3.4.6
nios2-linux-uclibc-gccbug
nios2-linux-uclibc-gcov
nios2-linux-uclibc-gdb
nios2-linux-uclibc-gdb66
nios2-linux-uclibc-gdbtui
nios2-linux-uclibc-gprof
root@ubuntu:/home/ramzan# nios2-linux-uclibc-gcc -v
bash: /opt/nios2/bin/nios2-linux-uclibc-gcc: No such file or directory
root@ubuntu:/home/ramzan# whereis nios2-linux-uclibc-gcc
nios2-linux-uclibc-gcc: /opt/nios2/bin/nios2-linux-uclibc-gcc

Thanks in advance ...!!!

First, you need to confirm that the file exists in your system through one of the two ways below.
Alternatively,
Should it be available, add it to your environment variables.

It is in
/opt/nios2/bin
i edited /root/.bashrc with this
PATH=$PATH:/opt/nios2/bin

but still same problem even i login and logout ....

you do not edit the /root account files because you DO NOT LOGIN AS ROOT

you edit the SYSTEM files ( or the normal user )
add that change to /ect/profile or to /etc/bashrc.local

root@ubuntu:/home/ramzan/nios2-linux# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/opt/nios2/bin


So the directory is in the root path. then why it is not finding the compiler

root@ubuntu:/home/ramzan/nios2-linux# /opt/nios2/bin/nios2-linux-uclibc-gcc -v
bash: /opt/nios2/bin/nios2-linux-uclibc-gcc: No such file or directory
root@ubuntu:/home/ramzan/nios2-linux# ls /opt/nios2/bin/
nios2-linux-addr2line nios2-linux-objcopy nios2-linux-uclibc-gccbug
nios2-linux-ar nios2-linux-objdump nios2-linux-uclibc-gcov
nios2-linux-as nios2-linux-ranlib nios2-linux-uclibc-gdb
nios2-linux-c++ nios2-linux-readelf nios2-linux-uclibc-gdb66
nios2-linux-cc nios2-linux-size nios2-linux-uclibc-gdbtui
nios2-linux-c++filt nios2-linux-strings nios2-linux-uclibc-gprof
nios2-linux-cpp nios2-linux-strip nios2-linux-uclibc-insight
nios2-linux-elf2flt nios2-linux-uclibc-addr2line nios2-linux-uclibc-ld
nios2-linux-flthdr nios2-linux-uclibc-ar nios2-linux-uclibc-ld.real
nios2-linux-g++ nios2-linux-uclibc-as nios2-linux-uclibc-nm
nios2-linux-gcc nios2-linux-uclibc-c++ nios2-linux-uclibc-objcopy
nios2-linux-gcc-3.4.6 nios2-linux-uclibc-cc nios2-linux-uclibc-objdump
nios2-linux-gccbug nios2-linux-uclibc-c++filt nios2-linux-uclibc-ranlib
nios2-linux-gcov nios2-linux-uclibc-cpp nios2-linux-uclibc-readelf
nios2-linux-gdb nios2-linux-uclibc-elf2flt nios2-linux-uclibc-size
nios2-linux-gprof nios2-linux-uclibc-flthdr nios2-linux-uclibc-strings
nios2-linux-ld nios2-linux-uclibc-g++ nios2-linux-uclibc-strip
nios2-linux-ld.real nios2-linux-uclibc-gcc tclsh8.4
nios2-linux-nm nios2-linux-uclibc-gcc-3.4.6 wish8.4

Given that the bashrc(~/.bashrc, /etc/profile, etc) files are read on login, you may have to manually get the OS to re-read them by executing for instance,
if you set bash as you default shell. Or simply log out and sign in again.

Try