best response confirmed by Jenny Malone (Occasional Contributor)
Solution

Hi Jenny,

 

After a couple  of rather frustrating phone calls and online chats regarding the same problem, as we were experiencing it too, one of the techs suggested it might have something to do with an issue they were having with OneNote online. It seems as though this may have been the case as all of a sudden yesterday, things started working again. That said any notebooks created through teams and stuck on that getting ready page, don't look like they are ever going to complete, and you may need to recreate the team :-/. Additionally for testing purposes I'd added a user to a Team with an embedded notebook and that user never duplicated to OneNote. Today, removing that user and readding them to the Team correctly added the user to OneNote as well.

 

Probably the most frustrating thing apart from it not working is that without exception, not a single person contacted through the official support channels, had no idea what Teams was. One of the support techs recommended I speak to someone in the Office Home support section. Obviously a crazy idea seeing as Teams isn't part of the home offering.

 

The TLDR version...it appears the link between Teams and OneNote was broken for around 48 hours, just not sure how widespread the issue was.

 

Cheers,

 

Paul