URGENT! Packaging failed due to unknown problem

Wow 's fix worked for me as well! " Open the Project Settings and find the “List of maps to include in a packaged build” option and press the + button to add an entry. In your case, you’ll need to add 4 as it seems you have 4 maps"

Unreal should really make this menu more evident. In Unity the packaged levels dialog is part of the build packaging menu.