The original owner should have cleared everything before selling the phone to you @lakjo
It's essential they do this so then the phone is ready to be set up as new with your own details.
You could also be hindered by the Samsung 2 step verification security protocols. Guide 2 step verification.
To be able to remove the Samsung Account you'd need the original owners Samsung Account Password as this is a security protocol.
Again with a factory reset.
Samsung Support wouldn't be able to help with this as they couldn't circumvent security protocols that are protecting the phone.
Can you reach out and contact the original owner for help.
Daily Driver > Samsung Galaxy s²² Ultra 256Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help.