A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
First, ignore the answer from Genevieve Cri please. An MS office repair would absolutely solve nothing and is in no way related to your question.
The Math solving feature is part of the OneNote App for Windows 10 only. It is not available to OneNote 2016 (the desktop or Office version). You most likely have both versions on your system, check the Start menu for "OneNote" instead of "OneNote 2016"
But: Some features (mainly in the App version) are only available to customers with an eglible Office 365 subscription; they will be missing from the app menu for others. The Math solver is one of those features. See the small printed second paragraph on this page: https://support.office.com/en-us/article/Convert-and-solve-math-equations-in-OneNote-for-Windows-10-1b37bb8d-ecd1-40d7-8d0f-5e6e46547441
First make sure you have the most recent version of the App. This is done using the MS Store App, not MS Ofifce update!
If you have an Office 365 subscription but still do not see the math assistant, try to add your O365 account to the OneNote App. (3-dot-menu, Settings, Accounts) as this is seems to be the only way for the app to verify your O365 account for now. There is no (technical) connection between the app and MS Office on your system; hence any Office repair or update would have no effect.