Looks like you messed up your PATH (export is a shell builtin). If it’s not in your .profile or .bashrc or so, quitting your current terminal and opening a new one should work. Otherwise, first try is to edit those files, or more simply make a copy of them and then remove the original ones. Start up a new terminal and see if that works: it should get the default system PATH. If it doesn’t, your system startup file may have been messed up (which would be very odd). I’ll need some more diagnostics if I want to help you further.