Your PATH variable seems to be messed up. If you are trying to run a restricted user with rbash, you should have /usr/bin in $PATH.
For a terminal session you can fix this by running following command:
export PATH=$PATH:/usr/bin
After that you can use sudo command, so you can edit /etc/environment file to make the change permanent, so, run in terminal:
sudo nano /etc/environment
To edit the file please make sure that the path is something like this:
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
Save and close the file with Ctrl+X and press Y when you are asked.
