I’m not sure what could be causing this, but a few quick questions whilst I do a bit of searching.
Firstly, just to clarify - your project is a Blueprint only project, correct?
Secondly, are you certain that your project settings (namely the Edit>ProjectSettings>Packaging page) are setup correctly, because this could certainly cause packaging issues.
Finally, does the Message Log offer any useful insight into the problem? If so, we could do with knowing it!
Good luck!
Edit: Just an after thought - have you tried a new, blank project, or one that you used to be able to package? That may narrow down whether it is this project or a wider problem with the engine.
Had enough of my questions yet? (last question - honest! ^^’ )