Iphone unlocked SIM not valid

Hi! My friends bought me an iPhone X unlocked from Best Buy US. The problem is that when i insert my Bulgarian sim card there is a message : SIM Not Valid - The SIM card that you currently have installed in this iPhone is from a service provider that is not supported under the activation policy that is currently assigned by the activation server. My question is when is unlocked how can i change the service provider to the one in my country? Thank you in advance