Can’t link accounts; address invalid

No. I’ve had service there for a few years, linking accounts is impossible. Seems like someone needs to do something on back end

We will look into it for you. 

Please send us a direct message with your full name and complete service address to "Xfinity Support". To do so, click on the direct messaging icon located at the top right of this forums page.

Here's the detailed steps to direct message us: 
How to Send Us a Direct Message:
To send us a direct message:
1. Click "Sign In" (If necessary)
2. Click the "Direct Messaging" Icon.
3. Click the "Start New Conversation" (pencil and paper) icon.
4. In the “To:” line, type "Xfinity Support".
5. As you type, a drop-down list will appear. Select "Xfinity Support" from that list.
6. An "Xfinity Support" graphic will replace the "To:" line.
7. Type your message in the text area near the bottom of the window.
8. Press Enter to send it.

For an example of how to send us a Direct Message, check out this link.
How to: Direct messaging within the forum | Xfinity Community Forum