Check if you have “Use UI Store” set to true.
This will package your project as a .pak, .ucas, and utoc file.
For some reason, when trying to create a patch, these files will cause this error.
Set “Use UI Store” to false and package again to only get a .pak file.
Now your patch build will have only a single _p.pak file that should work correctly as a patch for your base build.