I bought an iPhone that seems to be locked

How is it locked?

• Device disabled message —>If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
• SIM locked to a carrier: Contact the carrier it is locked to and ask about their unlocking policy—>How to unlock your iPhone for use with a different carrier - Apple Support
• SIM PIN lock—>Contact your carrier for the SIM PUK code
• Apple ID locked—>If your Apple ID is locked or disabled - Apple Support
• Device Activation Locked—>Turn off Activation Lock - Apple Support

If your phone says it is ACTIVATION LOCKED, unless you know the Apple ID and Password used to set up that phone, you have purchased a useless device. If you can't reach the owner who the phone is locked to, you can do nothing with the phone. Apple will not help you. We cannot help you. Either find the previous owner, or get your money back, or recycle the phone.

Read this about buying used iPhones --> The All Too Common SAD Reality of Buying … - Apple Community