Amber,
It should have given you an error if you left the path to OpenSees from the line edit. Does OpenSees exist on your system in the location shown in the image posted?
1) If yes, can you check OpenSees runs by trying to run opensees from a terminal window. If it fails to run, you will need to install it and provide the path to it. If it does run, we will need to set up a zoom call to sort out the issue.
2) If not, can you download OpenSees and provide that path to it.
frank