There are several things that you can do to try and fix this issue. Please follow these steps:
- If your Mac is using OneNote 16.25 or earlier, download and install the latest version of OneNote.
- Log out and log back into your account.
- Restart your Mac (by clicking the Apple symbol in the upper left corner, then clicking Restart).
- Verify that the default font and font size are set correctly in the settings.
- Bring up the ribbon (by selecting View > Show Ribbon).
- Select the font and font size from the drop-down menu in the ribbon.