An internal service error means something is set up wrong in the server. If it was just a file missing you would have a 404 error.
You don't have to have a robots.txt file to be indexed by Google or anyone. Bots would look for it and follow the instructions or if they don't see it they should go about their work. The trouble is that when they look they get the error.
Time to ask GoDaddy for some assistance. Good luck.