I found a used iMac and I can’t sign out

+3, yup you got yourself a brick. You can do the right thing though and have it recycled using one of Apple's no-cost recycling programs, just click Apple Recycling Programs and follow the directions.

We get similar posts to yours often on these forums. With a Mac unless the computer was setup for sale, give away, donation etc. per What to do before you sell, give away, or trade in your Mac the outcome is always the same, you got a brick. The reason being is the computer is still assigned to the original owner and unless the original owner executed these instructions to sign out of all of their accounts, the lucky finder (you in this case) didn't get something for free, they just got a liability.

basically, you found a "brick" in the garbage. without the password of the original owner, there is nothing you can do to get into it. no-one here can help you with that. and apple won't help you unless you can prove it is your Mac. for that, you would need the original bill of sale that must be in your name. sorry.

you can drop it off at an Apple Store and they will recycle it for free.

You must have the original owners password to move forward and do anything with that iMac.

If you do not have that password, then the iMac is useless and you should return it to recycle bin.

Lastly the original owner should have recycled the iMac properly instead of putting it into the garbage.