Unlock or domestically unlocked?

If you have iOS 14, go to settings/general/about, scroll down to carrier lock. If unlocked you’ll see “no SIM restrictions”. Otherwise the only way to know is by putting a SIM card from a different carrier in the phone.

I tried that already but I guess the SIM card is locked and didn’t request a PUK CODE! So what does that mean ? 

Tried what already? Why do you think the SIM card is locked? 

Bc I put it in my phone and it literally SIM card is locked please turn off the phone if it Persists so I did and it never asked for a PUK code 

Put what in your phone? A SIM card from a different carrier? Are you talking about an iPhone?

Yes a SIM card from a different carrier and yes I’m talking about an iPhone my mom has an old iPhone 6s which is currently on boost mobile and I want to give her my current old phone which is the iPhone XS from att and move it over to boost mobile 

The phone being locked is different than the SIM card being locked.  As described above, you can see whether the phone is locked or not by going into the phone settings.  If the Carrier Locked line says “SIM locked”, that means the phone is locked to a carrier.  That doesn’t mean that the SIM itself is locked.  If you’re asked for a PUK code, that means the SIM is locked, and you have to take that up with the carrier.

Ok this is what the message I got says! The SIM card has locked. Please power cycle the device and if the problem persists contact customer service

Where exactly are you seeing this message?  The Carrier Locked line says either “No SIM restrictions” if the PHONE is unlocked, or “SIM Locked” if the PHONE is locked to a carrier.  It’s poor terminology on Apple’s part, but it doesn’t necessarily mean that the SIM itself is locked, and the phone isn’t locking the SIM card.  It means that the PHONE is locked to certain SIMs.  In that case, the PHONE needs to be unlocked.  The SIM itself is probably fine. It just won’t work in the phone because it’s a different carrier.

Get the phone unlocked.