Cannot add cellular plan to apple watch

Hello, let's help get your Apple watch connected to a plan @waryman.

Thank you for the helpful information @formerlyknownas. In addition, If the 6 digit code is not working we encourage you to complete the NumberSync set-up to use your watch on the AT&T cellular network with your iPhone number. Lets try a few troubleshooting steps to get you back on track.

1. Open the Watch app to start the pairing process and follow the prompts.
2. Select Set up Cellular button.
3. Enter your AT&T user ID and Password. This should be the user ID that is associated with your iPhone number. If you don't have a user ID, click "create one now".
4. When you place a 911 call using your watch, the call will go over the cellular network when possible. If no cellular network is available, device location data from nearby Wi-Fi networks will be used to help locate you. If that data isn’t available, the address you provide will be used.
5. Make sure to check the box and read the Important Information for NumberSync on the 911 screen.
6. Select Verify Address.
7. On Sync requested screen, you should see a message indicating NumberSync should be available shortly. Select Ok.
8. Follow prompts to finish setting up your watch. 

The Apple Watch has an AT&T telephone number and associated rate plan and will appear on your monthly bill. Try signing into your account first and then:

1. Go to your device settings.
2. Tap one of the following (what you see depends on your device):
• AT&T Cellular Plan Ready to be Installed, then Continue
• Cellular, then Add Cellular Plan
• For step by step troublshooting visit our Device help center, select your device and then get started. 

Let us know if the steps were helpful, and we'll be here if you have any questions. Thank you for contacting AT&T Community Forums. 

Alasani, AT&T Community Specialist

Are you on a postpaid plan?

Which one?

Yes a post paid plan, the highest tier that is available. 

Numbered Instructions start part way down the page... 

https://www.att.com/support/smallbusiness/article/smb-wireless/KM1225992/