What if you bought the iPhone from someone

Make sure s/he unlocks it for you with his/her carrier.

Apple do not unlock iPhone only the carriers that locked can officially unlock.

Also read this important info below on what the seller needs to do:

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

Hi

Only the network provider iPhone is locked to Can remove the simlock.

If iPhone Is locked to an unknown Apple id get your money back.

Cheers

Brian 🇬-1F1E7;