iPhone Carrier Locked

The only way out is to contact the carrier it is locked to and ask them to unlock it.

Note: if you are not their customer, they are under no obligation to unlock it. You may be better off returning it and getting your money back if you can.

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

Return the phone.

It is not likely you will locate the carrier and they are surely not going to unlock the phone if you are currently not their customer.

You are taking a huge gamble purchasing a used device on Facebook Marketplace. Buyer Beware!

The All Too Common SAD Reality of Buying … - Apple Community

And what did your carrier say when you contacted them about their service?

That is an issue only the carrier can solve. Currently that cellular service is not activated on the phone.