Hi AdminPerson and welcome to the forums! This link will walk you through how to link or unlink accounts: https://business.comcast.com/help-and-support/accounts-billing/link-unlink-comcast-business-online-a... . If you have trouble or questions about any of it, you can also reach out to us via private message with your name, the business name, the complete service address (including city, state, ZIP, suite number, etc), and the phone or account number by clicking on my handle (Comcast_Gina).
Are you still receiving the error when you attempt to private message us? If you go to the top right you should see your forums handle with an arrow pointing down next to it and if you click it should open a drop down menu with a Help option that will go into a bunch of things, but the one that's needed is the Private Messenger information. It has instructions on how to send, reply, etc. Since you are the Primary Manager, I'd like to check the accounts and why they are not letting you link them.