Welcome to the forum.
The community would like to help but can you clarify a few items for us?
1. When you were activating the MyAccount did you create a ".comcastbiz.net" user account?
2. Are you using the primary user account while trying activating SharePoint?
I am using https://businessclass.comcast.net. I only have one user (primary). Under the websites menu I created a domain (which is what the help said to do), but the sharepoint option did not showup under the internet menu(as also shown in the help). As for ".comcastbiz.net", I don't know what that is.
Thank you for the information.
Based on it, I was able to find that your account was activated using the BYOE (bring your own email) process.
Bring your own email (BYOE)
Bring your own email (BYOE) allows customers to utilize a private email address to view and manage their billing accounts online. This feature however DOES NOT allow customers to manage any of their Business Internet Services including; Web Hosting, Add a Domain, SharePoint, Exchange Email etc. or Business Voice Services. To enable management of these services, you will need to use the create new email address option (below).
Create New Email Address option
This option MUST be selected to manage Business Internet services and allows access to advanced features like Web Hosting, Add a Domain, SharePoint, Exchange Email etc. It is necessary to create a comcastbiz.net parent domain in the form of <yourbusiness.comcastbiz.net>