Icant use a SIM card at this phone.what I’m going to do

Contact Apple using the Contact Support link at the top of this page. Ask the Apple tech which carrier the phone is locked to (you will need to provide the serial number or the IMEI of the phone). Then contact that carrier and ask about their unlocking policy.

What is the error message you see when you insert a SIM card?

And what model phone?

You're going to get a sim that is supported by the carrier you have on your device.

Call them and ask them to give you one.

It's their job.