I just purchased an iPhone 13 and it says "SIM Not Supported upon" setup. Here are more details it provides
"The SIM 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 install another SIM from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information."
@JaredChunesingh where did you buy the iPhone? was it from a trusted source? and was the phone from US?
When your iPhone says SIM is not supported, it usually means that your second-hand iPhone or even a new iPhone is locked to a carrier.
You can go to Settings > General > About > Find Carrier Lock and check if it shows No SIM restrictions, then your iPhone is unlocked., otherwise, itis locked