What to do when your phone says carrier/sim 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.

By the way iPhone 6S is > 6 years, normally (in the EU) after 2 years the consumer has the right to ask that the carrier unlock the iPhone. So ask your Carrier.

It shouldn't be complaining about carrier lock unless you are trying to change phone service, which you apparently are not doing. That makes me think there's some kind of error inside the device making it think you are changing service. Check you carrier is set correctly. Refer to this support article to learn how to manually update your carrier settings on your iPhone or iPad --> Manually update your carrier settings on your iPhone or iPad - Apple Support

Unfortunately your phone is now considered to be vintage by Apple so while you can free free to contact Apple about this, they may not be willing to service the unit should there be a problem.

Try this -> remove and reinsert the SIM card.

If it doesn't help contact your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

Have you contact / visit your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

Are you trying to switch to a "new' carrier? If yes, you need to contact your "old" carrier and ask them unlock the SIM card on the phone.

If you are still on the same carrier, you need to contact your carrier for help to unlock the SIM card.

VISIT your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.