User “Tim” posted in the Verizon Community Forums that the issue has to do with whether or not the Apple Watch was purchased through Verizon or whether it was purchased from another source. A Verizon-provided Apple Watch generally has no issues in being activated on the Verizon network.

There are growing reports that many unlocked Apple Watch Series 10 users are running into Verizon activation issues.

Verizon subscribers who purchased their Apple Watch somewhere other than Verizon are reporting activation issues when attempting to connect it on Verizon's cellular network.

Apple Watch owners can't activate their cell plans through Verizon — unless they bought the device from the carrier, or complain at length.

As noted by AppleInsider, Apple Watch owners can’t activate their cell plans through Verizon — unless they bought the device from the carrier, or complain at length. And I can attest this is true. App

Some Apple Watch users say they've been unable to activate the cellular plan for their new wearable. Verizon suggests they try again.