OK World Streaming test, make a new Miscellaneous>HLOD Layer in the Content Browser and drag it onto the Default HLOD layer slots in World Settings.
Do not build HLODs, keep it empty and try to Launch.
If your launch is successful it’s a HLOD layer issue
If your launch is unsuccessful it’s something else, try then unchecking World Streaming in World Settings and launch.
Successful Launch, it’s most likely an issue with something to do with spatially loaded things
Unsuccessful launch, it’s something else, like an invalid reference in a device or verse… or a rogue prop.
How did you go ?