How do I remove an account or unlock an iPhone 17 Pro linked to someone else's account?

You can’t unless you have the original receipt to show Apple

How did this iPhone come to be linked to someone else's Apple Account?

Did you buy it second-hand, only to discover that it is Activation Locked? If so, see

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community

Did you fall for a scammer's lies, and log into their Apple Account, only to have them lock your own phone against you? If so, losing your phone forever might be just one of the consequences of that fatal mistake.

Signing Out of Your Apple Account and Into Someone Else's Account Could Have Catastrophic Consequences – Apple Community

We may not be getting the whole story here.

Did someone trick you into logging in on your iPhone using another Apple Account and Password?

If this is what happened, you have turned over control of your iPhone to someone else. Your iPhone is now useless to you.

Or, if you bought a used iPhone that is Activation Locked, the phone will be useless to you unless you know the Apple Account Name and Password of the person who originally set up the iPhone.

If you bought a used phone, and the owner parted with it willingly, they should have done this:

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

If you bought a used phone locally, from a friend or relative who simply overlooked this, and who is willing to help you, you could take the phone back to them, and ask them to carry out those steps, then return the phone to you. That would wipe everything on the phone, and the phone would then open up to the "Hello" screen, so you could set it up as if it were a new phone.

If you bought a used phone from some seller who "disappeared", "ghosted" you, or refused to help you at all once they got your money, it would be a rather different story …