I contacted support via email and they got back to me requesting these three things...
• A screenshot of the error messages you’ve received
• Pictures or scanned images of your proof of purchase for the iPhone
• The version of IOS that you phone is currently running
I replied saying I couldn't supply the first as I no longer got the message and the only option was for me to try free for 1 week. I supplied a copy of my Amazon invoice, and also my version of iOS along with my iPhone serial number just for good measure.
Yesterday evening I received a reply with apologies and a code with instructions so that I could redeem my free year. I followed my instructions and used the code today and my Apple TV+ free year is now up and running 😊
If anyone is still experiencing this issue, then I'd suggest contacting support and hopefully you'll have this fixed too.