Did you insert a Verizon sim to a Verizon version of the phone?
An important distinction this year is that there is no such thing as the AT&T/T-Mobile model and the Verizon/Sprint model as has been the case in the past. This year, every iPhone XS, XS Max, and XR sold in the U.S. has identical hardware regardless of which carrier you pick.
The only differentiating factor between the phones sold in the U.S. this year is whether they are unlocked or not. If you buy an iPhone from Apple and you pay for the phone in full, the phone you receive is unlocked. If you pay for the phone using Apple's installment plan (the iPhone Upgrade Program), the phone is also unlocked.
However, if you choose to have the device installment payment tacked onto your monthly cell phone bill instead, you are on your carrier's installment plan and the phone will be locked to said carrier until the device has been paid off and an unlock has been requested.