In your Source directory, there needs to be a “target” file for each build type you want (that means whether it’s building the editor, game, or a server).
If you need an example, you can create a template project and look in that source. The files will be marked *.Target.cs.
If it worked before then those files must not have gotten copied over somehow. If they’re gone for good you can recreate them by copying from a template project and renaming the files/classes so they match your project name.