The catalog seems to be an exported catalog or has been copied

When trying to migrate from LR classic to LR CC I get this message:

"The catalog seems to be an exported catalog or has been copied"

I know it is a common issue and here the workaround says to just open the catalog back in LR Classic. 

Problem is, I don't have LR Classic anymore as I updated to Catalina and changed my MacBook thinking that having the catalog backup and the original raw files would be enough. Any suggestion is welcome.  Thanks!