Why can't I open a project file from NVivo 12 in NVivo 15?

If you are on Windows, you should launch the older version of NVivo (if possible) without opening the file and go to File->Help->Compact and Repair and select your file in the Browse window.  Once this completes, open a new, blank project file and import your file into it.   If you are on a Mac, please launch the older version of NVivo (if possible) without opening the file and go to File->Compact Project and select your file in the Browse window.  Once this completes, open a new, blank project file and import your file into it. If you no longer have access to the older version, in order to convert an older NVivo Mac file to a newer NVivo Windows format, the file needs to be converted to the older NVivo Windows format first. If you no longer have access to the older NVivo Windows version, log into the Lumivero Community Home Page and click on the Submit a Case button to ask an NVivo analyst for assistance.