The “Classic,” “Legacy” Audacity would not allow you to edit or change a Project outside of Audacity. So the Project you edited is probably trash.
You can’t change the name of a Project, either.
Open up the Project AUP file in a text editor and read it (don’t save anything). Somewhere between two and four text lines down, there is an entry with the original project name. Both the AUP and the _DATA folder have to have this name.
Now it gets magic. I thought Audacity 3.0.0 used the new Project format that uses one AUP3 file and no _DATA folder.
Somehow, I think you’re crossing the older AUP Project format and the AUP3 format. I don’t think the AUP3 file will ever complain about not being able to find its _DATA folder because it doesn’t use one.
Audacity 3.0.0 will not make an AUP file, but it should open all Project formats, including the older one.