The photos and videos have now been uploaded to iCloud and are showing in the Photos app on my Mac. So, from this perspective, problem solved. Because they were large files, like 4K videos and I was looking for a quick edit, I then transferred all files via Airdrop, which I don't usually do. The HEIC files came straight from my iPhone with no third-parties apps being used. They were automatically saved in the Downloads folder, which I then transferred manually to the Pictures folder. When I tried to import these files from the Pictures folder into the Photos app, I then encountered the problem and got the incompatibility messages. So, I haven't managed to find the answer to my original question, but the problem has been solved nonetheless. Thanks for your help and input anyway. All the best.