password not working on a new(old) iMac 2015

This sounds like the seller failed to properly prepare the Mac for resale. What should have been done by the seller is here:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If the seller cannot remedy his oversight, you should get a full refund.

Then it could be that the password that is taped on is not the correct password. As the methods that can reset the password most likely need an Apple ID password or a recovery key, please contact the seller for help

If the Mac was properly configured for resale you would have gotten the famous Hello screen in multiple languages when you first turned on the Mac.

If you didn't see this it wasn't prepared properly and you should return it.