I'm seeing this same issue, Xcode 12.3, Deployment target iOS 14.3. I can confirm that the problem happens even when creating a new project in Xcode, using the default game project selection. Super frustrating.

Ah, after some trial and error I found the solution. There needs to be a Launch Screen File selected. Click on the Project, then select your game under Targets. In the General tab, scroll down to "App Icons and Launch Images," then for Launch Screen File select the Main storyboard (assuming you started with the generic Game project template).