iPhone sim not valid

Are you the original owner of this phone? If you are contact your carrier and if necessary get a new sim card.

Are you trying to change carriers by simply inserting a new sim card? Your phone is likely locked to another carrier. Contact that carrier to see how you can have the phone unlocked and if you qualify.

If neither describes your situation, please provide more information.

It means you need to contact the carrier the iPhone is locked to, to unlock it.

Only the carrier which locked iPhone can officially unlock. Not Apple or anyone,