App linked to old, inactive apple-account

Bought a new Mac. First in a long time. Created a new Apple-Account. Everything is set-up with this new account. Wanted to install the free Garageband-App and somehow it tells me that the app is linked to an old apple-account of mine, asks from me to log into that old account, with an email-adress I don't have access to any more and I don't have a password for.

I can't log into that old account or even reset the password, because the system tells me, that this account ist not supported.

How does the system even know that that this old account belonged to me?

I find no way around this.

Please help.