I can't reset my old iPod (6th Gen)

I found my old iPod and my fingerprint is still there but I can't get in because I need a passcode, which I forgot. The device isn't listed under my Apple ID anymore for some reason and because of that I can't do the 'trust' connect method with my Mac. Because it's old I can't find the invoice or packaging or anything that can help me find a serial number. Nothings working. What can I do? Can I ask for an invoice to be resent so I can find the serial number? Or can Apple reset it for me? But I still wanna keep the data that's on there, even though that's probably impossible. Please help.