when on my mac - and trying to open photos - I get the error: ncannot find "photos library.photoslibrary" . This file is not on my mac. What to do? All my photos are on icloud and I have them on my iphone and ipad. how do I link up my mac again?

Do you have the iCloud Library on your iCloud and did you have iCloud Library enabled on your MBP previously?

If you do you can create a new Photos library on your Mac, designate it the System Photo Library and enable iCloud Library. That will download all of the photos from the iCloud Library to your new Photos library. If you don't have room for a full sized library on your laptop then select "Optimize Mac Storage" on your Mac's library.