A few months ago people were up in arms about iPhone backups not necessarily being glued to one computer forever.
My iPhone is backed up to a computer that I no longer have access to.
How do I convince my iPhone and my new MacBook Pro to get along and back up the iPhone?
I ran into the very issue today (assuming you want to make an iTunes backup). I found an apple Support page that solved this for me. You have to download iTunes version 12.8 (not the updated 12.8.something) and connect your phone. You will be asked to do an update before the phone can sync and once the update is done the phone syncs.
It’s very very Rube Goldbergish and very very non-Apple like.
I’ll root through my history and see if I can find the page I used.
Well, that didn’t take long. I am still on High Sierra, so this is what solved the issue for me.
If you are on Mojave, you may need to try something else.