loading photos library into new macair PhotosErrorDomain error 4302

Can you give us more specific information? I'm not sure what "load photos library folder into Photos" means. Normally, the Photos Library package stays in the user's Pictures folder on the Mac's internal drive. Is that the case for you? Normally you can open a Library by double clicking on it. Where did this "photos library" come from that you are moving? How are you moving it?

The Photos Library can be in any of any of a number of places, but it can't be in iCloud Drive or on any networked drive. The drive must be connected to the computer by a wire cable. And to avoid damaging the Photos Library a drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

In fact, this is a common cause of Error 4302.

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

Also, let us know the status of your use of iCloud Photos and "Optimize Storage."