Have changed my 2 Apple Watches. My first one set up fine; my second one has been stuck on error 08 (…..please allow 24hours for activation…..) for the past 5 days. Reset/unpair etc doesn’t change anything, and O2 have said there’s a reset stuck on the data plan sync and will be resolved within 24hours….which they’ve been saying now for 3 days each time I get an update.
Your old watch needs to be removed from your account.
I'll ask @O2Lisa to help you.