Dear respected Brianna Nation,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
I’m sorry you’re having a problem but based on your description we can work together to resolve it.
For the situation you encountered, we deeply understand the inconvenience caused and apologize for it.
As per your description, firstly, may I know are you using OneNote 365 app(formelly called onenote 2016 app) or OneNote Windows 10 app or OneNote for web app?
Also, if possible, could you please upload screenshot for our reference, so we can understand your exact scenario and provide our suggestion in a productive manner, perhaps some related screenshots will be helpful.
Meanwhile the Add a section group feature available in OneNote 365 app(formelly called onenote 2016 app), when we Right-click any section tab, and then click New Section Group.
Note: To perform above steps, you need to have OneNote 2016 for Windows application. Refer to this article to install the OneNote 2016 for Windows application.
Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution.
I'm sorry and I apologize for that.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad