You’ll need to add /home/user-name/.fly/bin to your $PATH variable for your user, which in Ubuntu is typically something that can be set in $HOME/.bashrc or $HOME/.bash_profile.
It sounds like maybe(!?) something is wonky with your Ubuntu installation (or we’re missing some piece of the puzzle), but in general adding to $PATH is like this in Ubuntu: environment variables - How to add a directory to the PATH? - Ask Ubuntu