iCloud / Photos and iMac - issue accessing "“Photos Library” is in iCloud Drive, and needs to be moved to a folder on your Mac to open."

I have a trusty iMac and 15 pro Max. I take a lot of photos (around 75,000) stored on my iPhone and into Photos on my iMac.

Today, just logged into iCloud from a Browser (Safari), to check all was well with photos etc., going into the cloud. Now I am getting this !!!

“Photos Library” is in iCloud Drive, and needs to be moved to a folder on your Mac to open.

To make matters worse, I need to port over all my data from my old iPhone to a new iPhone that is arriving today as I have promised my old iPhone to someone else tomorrow. This is now causing real issues as I am in time constraints so any assistance would be gratefully appreciated as to how I can ensure I can access my large (75,000) photos on my iMac, iPhone and iPad

Thank you very much in advance

Debs