New Hard Drive and Apple id issues on Itunes

I have had to have a brand new hard drive installed in December 2025 into my iMac which I have owned since 2013 I manged to recover some of my information because I had a external time machine which I backed up to. However I did not realise the IMac had stopped backing up to the external time machine at the end of Aug 25. Everything is working OK but I lost a lot of information which I have to live with.

However I have an issue with my Itunes every time it backs up to my ipad over wifi I get a message saying "this computer is no longer authorised to play purchased items that are on the ipad"............ There are 173 tracks on the ipad which its saying it will remove if I do not authorise the iMac.

Then I select authorise,

Then it ask me to enter the password for an apple id showing a e mail address I have not used in over 20 years before i even got the IMac the i pad is 10 years old.

If I then enter this email address and say I have forgotten the password it then says the "apple id is not valid or not supported"

I have no idea what this password is and before the iMac I had a desktop Microsoft computer with no music on it. Any help would be welcome