HELP

I just bought a Iphone XR from my cousin she said it was unlocked and that my sim card should work in it but when i got it said " The SIM card that you currently have installed in this iPhone is from a carrier taht is not supporeted under the activation policy that is currently assigned by the acrivation 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. Pleace cotact Apple for for more information." But she told me that my SIM card should work in it becasue it was unlocked.