Hi Krithaca22,
Thanks for using Apple Support Communities. We understand you are running into an issue setting up your new Apple Watch. We're happy to help. It sounds like you are trying to transfer your cellular plan to your replacement Apple Watch. If so, make sure you're using these steps to complete that:
1. Remove the cellular plan from your old Apple Watch. You can do this from the Apple Watch app or by erasing your old Apple Watch.
2. Pair your new Apple Watch with your iPhone. During setup, tap “Set up Cellular” to add a cellular plan.
3. Some carriers allow you to transfer your existing plan to your new Apple Watch directly from the Apple Watch app.
4. If you don’t see an option to transfer your cellular plan, contact your carrier for help.
See more about those steps here: Set up cellular on Apple Watch - Apple Support
If you're still running into an issue after connecting with your carrier, let us know if you've made any changes to your cellular plan recently. We'll go from there.
Thanks.