I bought an iPhone but it is SIM locked

That's a US phone. You did NOT buy it from an authorized iPhone distributor.

Return it and get your money back.

See this Apple article: If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

• Contact your carrier
• If your carrier confirms that there's no issue with your account or network, but you still can't connect to a cellular network, contact Apple Support.

See also this Apple article: How to unlock your iPhone for use with a different carrier

How to unlock your iPhone for use with a different carrier - Apple Support

• Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.

Or do you need the PUK code?, see this Apple article: Use a SIM PIN for your iPhone or iPad

Use a SIM PIN for your iPhone or iPad - Apple Support

If the problem persists, contact that retail apple store In Nigeria.