andrewfromeast wareham wrote: … seeing pop up message "Photos cannot load edits for this image". What is the cause and remedy?
Well, we can't know exactly what's going on without interacting with your computer. Maybe you could give us more information? For instance, do you use iCloud Photos?
Do you have Photos using Optimized Storage?
How much free storage do you have on your Mac?
Where does your Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
That sort of message occurs when Photos has lost the connection with the original picture files. Photos normally operates a "Managed Library." That means that the original picture files are loaded into the Photos Library so Photos can keep track of them. Photos can also have a Referenced Library where the Original picture files are kept in Finder folders and are not added to the Library at all. A Referenced Library is really for testing purposes, and accidentally creating a Referenced Library has caused lots of troubles for people. Almost no one intentionally has a Referenced Library. If Photos loses contact with the Original files, then you may be able to see the Thumbnails or the full screen Preview versions, but you won't be able to edit or print the lost pictures.
In Mac Photos Settings>General, "Copy Items to the Photos Library" should be checked.

Have you ever had this box un-checked? That would mean that some original files are in Finder folders, and they may have moved since the migration.
In Optimized Storage, Photos saves space by keeping many of the Original picture files at iCloud.com, and it reaches out to iCloud when you need to edit them. So if you are disconnected from your iCloud account, then editing isn't possible for those pictures.
Can you tell us the details about your setup? Do you know where those original picture files are, or can you find copies of them? Do you have backups?