Build Failed: Error Launch Failed! Unknown Error.

I am not able to reproduce this error when creating a first person template project with UI in it. Could you please check to see whether or not 4.14.1 works for you?

Have you tried migrating your project over to a newly created project to see if that fixes the issue too? I would not replace the old version with new until you’ve verified it’s corrected the problem. Sometimes older files will throw out errors.