Sim not valid

You will need to call AppleCare, give them the serial and model number and tell them you just want to know which cell service provider your iPhone is locked to. Then contact that cell service provider and ask what their unlock policy is and if you qualify.

There is no "solve code", YOU have to contact AppleCare to see you your iPhone is locked with (call them or go by an Apple Store or Apple Authorized Service Provider) and then YOU have to contact that cell service provider. iPhones do not use a code to unlock.

This is a user-to-user technical support site, Apple technicians are not here and Apple cannot start the unlock process anyway, the cell service company it is locked to has to do that.

Why don't you ask your friend which US cell service provider they used it with? That is going to be the one it is locked with.

Click on the Contact Support link in the top right of this page to connect with Apple Support but your friend should be able to answer the question.

Yes.

Your phone is carrier locked. Only the carrier it's locked to can unlock it. You will have to contact them.

that means that phone is locked to a different phone carrier.

Are you in Saudi Arabia? Where did you buy this iPhone?