Lightroom cannot use this catalog

[BrianATaylor]

was the message I received when I tried to launch Lightroom.   I have done my best to make sure lrcat.lock files are not present.   It actually, won't allow me to launch any of my previous catalogs.   My catalog resides on an external harddrive.  Lightroom is on my Mac.  I'm at a loss on where to proceed  

{Moved from Lightroom Cloud to Lightroom Classic Forum by Moderator} 

[cmgap]

Please upload a screenshot of your  Lightroom folder.

[GoldingD]

What version of LrC are you running? 

Current is v10.1

Did you perhaps roll back from v10 to v9?

[BrianATaylor]

If I did accidentally roll back, what would be the fix? Thanks.

Sent from my iPhone

[ManiacJoe]

Along with answers to the previous questions, what is the exact wording of the error message?