Sim card is not supported on iPhone XR

Hi,

I just bought an iPhone XR from my friend. It was on AT&T. When i try to set it up with my T-Mobile Sim card, it says:

"SIM Not Supported. The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iphone be unlocked by your carrier. Please contact Apple for more information."

How can I fix this? Can I take it to the T'Mobile store or do I have to get my friend to take it to the AT&T store?

Thanks for any help.