Q: Sim lock

Hey its depend on your iPhone which one you purchase if you buy old one that there is be issue to insert new SI understand all about SIM

https://support.apple.com/en-us/HT201529

If your iPhone is locked by a carrier, you need to contact this carrier to unlock it. Note that this is only possible when the contract has been already terminated and there're no charges left to be paid.

Yup. The phone is carrier-locked and the SIM you put in is not from the one it is locked to.

The only possible solution was already given to you. Contact the carrier in order to unlock it. Unfortunately no one here can help you with this.