I have purchased an used iphone 8 from a third party

This is what the 3rd party should have done......before....you received the phone.

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

They did not do this, so your phone is in Activation Lock.

You will need to get in touch with the seller to have them remove the lock. The phone is useless until this occurs.

How to remove Activation Lock - Apple Support

Activation lock is a security protocol that can't be bypassed. Only the person it is locked to can unlock it.

If you bought this from someone and it came locked, try to get your money back, you will never be able to use the phone.

If your phone is Activation Locked - How to remove Activation Lock -

How to remove Activation Lock - Apple Support

The All Too Common SAD Reality of Buying a Used iPad/iPhone.

The All Too Common SAD Reality of Buying … - Apple Community

If you can’t get hold of the seller, or they can’t or won’t remove the activation lock and will not give you a refund, turn it over to the police. In that case it’s likely stolen.