Authorised computer won't sync

I inherited my hubby's iPhone6 a few weeks ago, and had no problem setting up and syncing with my MacBook Pro at the time. Today I've wanted to add a new file into iTunes from my computer, then went to sync to my phone and got a message to say that my computer isn't authorised. However it is actually authorised. I have de-authorised and then re-authorised, I've checked it through the iTunes store. I've gone in through the administrator account on my computer.

Would love any other ideas, as I'd really like to be able to sync this new file to my phone, and do a backup of my phone.