SIM Not Supported error on iPhone X?

Hello,

I inserted my Verizon sim card into my iPhone X and received the following message: "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 a 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"

I got this phone from a friend and it was originally on Sprint. Sprint said you should be able to take it to any new carrier and it should work. The phone was paid off when the contract with Sprint was terminated. Sprint said it can be used on any CDMA/GSM service domestically or internationally.

Any thoughts on what to do?