Keep running (start and stop) it until you see all tests pass and the page with the form stays up. Once you get it to pass and have the page still accessible then submit the project.
If that still doesn’t work try forking your own project and use the fork (sometimes forking the project changes something, don’t ask me why).
Edit: Just to be clear, the issue is with Replit and not the tests. If you can not get it to work using Replit another option is to try using Glitch. You can import the GitHub starter repo when creating a new project on Glitch. Then copy over the code and try submitting again.
Edit: Also, can you please look in the network tab when submitting, see if there is anything in red. Post a screenshot as well if you see anything in red.