I have an iPhone 6s plus and the sim is locked

See 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.

👉 You need to contact your Carrier.

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

You will need to contact the carrier that the SIM is locked to for help. Apple cannot help you.