The error says very clearly what’s going wrong. You’re trying to package up something called the ImpostorBaker, into your shipping game.
It sounds very much to me like this is an editor tool, which shouldn’t be marked as included for shipping/packaging.
Well, it’s not part of the standard or empty project templates, so it must have come from some additional content or plugin you have downloaded or purchased or installed or enabled.
Try this:
Create New Project
Choose First Person Template
Save the default level
Choose “package → Windows”
Does this also generate the error? If it does, then something has been installed/mutated in your engine installation. If this works, then the problem is in your project setup.