Call or chat with a customer service rep. Ask the agent to delete your watch from your account. When you go back into the system, it should allow you to add with no problem. I have no clue why this works but I saw it on another forum for a different phone company when they were experiencing the same issues.
I had moved through the frustrating tier 1 and tier 2 processes. Including two passes through Apple which I doubted was the problem since it seemed the Apple Setup Cellular routines successfully hands off to the Xfinity setup app which is where everyone gets stuck. Once Chris got involved, Chris figured out what was going on with my watch/account and made sure it worked. Not enough gratitude for @ComcastChrisL who dealt with me as a person. Very refreshing after the previous frustration.
As I understand it, for me there was an account setting at the host's end that needed to be reset so they could establish and send an eSIM. It seems the problems differ so I can't say this is what you are experiencing.
I hope the support team can identify all the problems and fix thier setup app and backend systems it taps into.
Apple Watch Series 5 Ceramic, purchased from Apple, never before activated on cellular, same issue with Uh Oh screen..