First, when you open Ubuntu, try to run thisroot.sh, which is in the root/bin folder, i.e.:
source /path/to/root/bin/thisroot.sh
(use the path where you extracted root!).
If that works, you have to run that every time you open a new Ubuntu terminal window. To do it automatically, add that same line at the end of your ~/.bashrc file. If it didn’t work (and you are sure the file thisroot.sh is in that location), it may be better to reinstall root. Follow the instructions for WSL.