I'm in the same boat with my brand new Google Pixel 3a, unlocked, straight from the Google Store. The IMEI won't validate, and even XM tier 2 support says "it's not a Verizon phone," which is impossible because the Pixel 3a runs on all networks.
Verizon's own IMEI checker validates it and offers to let me activate it. Crazy.
An employee has asked me to contact them directly in the thread I started; I have done so. Something's wrong on the XM backend. They did just add Pixel support for BYOD, so this frustration isn't a surprise.