Can't Create a Teams Tab for Existing OneNote Notebook
This article applies to: Microsoft Teams
If you are trying to add an existing OneNote notebook to a team and are not presented with the option, it may be because the team was created from a template that doesn't provide it. When creating the team, selecting Other as your team type will let you create a tab for an existing OneNote notebook later.
There is not a way to change the template once the team is created.