Hello there @McIsCool75!
The error you are getting aims at the presence of a level blueprint (or a reference to one) in your project, which is not directly supported in UEFN. If you were working in regular Unreal Engine, this would be ok. But in UEFN, all logic should live in device and Verse.
The first approach would be to remove any of these references from your project. “LevelScriptBlueprint” is usually in the World Settings section of a map, so check said tab in your level (should be at Window > World Settings), then look for any object with the “Blueprint” term. If you find any, try to clear them, or set their assigned value to “None”.
Since the issue wasn’t happening before, it could be related to the imported elements you mentioned (sounds and textures), there could be a BP ref mixed between. Use the Content Browser, and filter by Blueprint Class. If anything shows up that is NOT a UEFN device, open said element, remove any active references, then delete it.
Another option would be to create a new, blank level in your project, then pick all actors from “The Return” level in Outiner, and copy them to the new level. This should leave behind any stale metadata, or BP references.