Seems to be caused by the emulation dropdown. Since you checked "Emulation" you need to choose an emulator. But as you can't choose one, the required field stays empty. Contact the game mods and let them know about the empty emulator dropdown

^

The Emulation field was required but had no possible options which is a mistake on the side of the game mods, you did nothing wrong. I've updated the backend code to prevent such a state being possible. I've also made the error message a bit more informative.

Contacting the game mods is still the correct action, though for the time being the Emulation field is no longer required.