Photos downloaded from Icloud won't open

I have a a late 2015 Imac running MAC OS Sierra 10.13.6. For ages, I have been able to open Icloud, select the photos I want from my Icloud account and download them to my desktop. Last year, I got a new Iphone 15 but that worked fine before Christmas. Over Christmas, I had to do a software update on my new Iphone and now, I can see all my photos on icloud, but when I select the ones I want to download to my Imac, it transfers all the files, but when I try to open any of the photos on the Imac, it says :

The file “IMG_2456.HEIC” could not be opened. It may be damaged or use a file format that preview doesn't recognise

If I use my friends older Iphone, it opens the HEIC pictures with no problems and operates as it should.

How can I get around this issue as I need the pictures for work. Any help will be greatfully received.