Teams (and so Office 365) are created by the IT department. End users can't add new Plans to their Teams, even if they are the Owner of the team.
In my opinion this is unexpected behavior. Is this really intended?
EDIT: Apparently it's possible to add new plans to existing groups, that already have atleast one plan added. I guess there are some processes in the background, that add attributes/infos to the Office 365 Group mailbox and Sharepoint team site when adding the first plan to a group.