Sim
I recently bought an iPhone XR off a person I know and it says Sim locked and won’t let me add a SIM card
iPhone XR, iOS 14
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You or the seller will have to ask the carrier to release the SIM lock.
The carrier may or may not be willing to discuss this unlock with you, or may only be willing to discuss the unlock with the original contract holder.
The carrier may or may not be willing to release the lock, too. Some do. Some don’t.
If the seller has not finished the term of their contract with the carrier, then you’ve seemingly just taken over the remainder, or you’ll be returning the device to the seller for a refund if they’re still available and are amenable.
Or you’re using the carrier network, of course.
How to unlock your iPhone for use with a different carrier - Apple Support
Only the carrier holding the carrier lock can release it.