Q: Activation of old phone

You need the previous owner's Apple ID and password (actual, correct ones). Without that information, only he/she can unlock the iPhone. There is no other way. Not even Apple can help you.

These are the instructions that the previous owner should follow.

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

Hi,

The only option is to Contact your seller and ask his ID and password to reset the phone.

Bad news ... you were very likely sold an iPhone you cannot use.

It is Activation Locked, and will remain so until the original owner unlocks it.  If the original owner does not unlock it, you will never be able to use the iPhone.  It would make a nice bookend ...

Return it if you can.

Find My iPhone Activation Lock - Apple Support