I know this sounds like a pretty generic answer, but you'd be surprised how many random errors it can fix.
Try clearing your browser cache, restarting your browser, and coming back to GASR to see if you still can't highlight text.
Press ctrl+shift+delete if you use Windows (Command-Shift-Delete for Mac) and it should bring you to the window to clear it.
If that doesn't work, trying downloading/using a different browser. If you can highlight text, then you know it's something with the original browser and you may need to update/re-install it. If you still can't, post here and I'll keep troubleshooting with you.