What solved it was the following info from somebody I spoke to, they said "its a late 2015, should be able to run macOS 12.2 Monterey which is the most recent macOS, its just too old to get that update in app store"

this page has links to old versions of Mac osx https://support.apple.com/en-gb/HT211683

I "backed up" the computer with CCC / carbon copy cloner. Cloned the main partition to an external USB hard drive. So all the important data was there.I didn't test if I could boot off it but that was sufficient for me.

I went to download the latest one on that page, called "Big Sur / MacOS 11".. I think it ended up showing one later than that, Monterey / MacOS 12. So I installed that one.. (I was told it was a bit of a risk to go straight to that as it was a big upgrade and Monterey is very new, so worth doing to Big Sur first, wasn't entirely intentional, but anyhow..)

It's for a friend and they have a lot of apple software configured on other devices so if there were any issues reconfiguring any software then that info would be on the other working apple computers they have. Turned out didn't have to reconfigure anything anyway.

When updating , when it said 1 minute or so, that had to be left overnight to complete. I don't know if that's normal for apple. But then was fine. I've seen an update take days before.