I bought an I Phone 7 off offer up and they told me they forgot the icloud they used and so they couldn't unlock it
What do I do
Return it. There's no way to bypass the Activation Lock if you are not an original owner.
If you can't contact the person who sold it to you, it's basically a stolen device, because the owner can remove the lock even if he forgot his Apple ID. Then bring it to the police station.