If you have trouble opening a notebook from a SharePoint site on your Android phone, it could be because of any of the following reasons:
You might be using the notebook’s short URL, which is not supported on Android phones. You can only open notebooks showing a complete SharePoint URL, which will look similar to this:
Tip: If you don’t have the complete URL, try opening the notebook from a non-Android device.
You are accessing a notebook that is stored on an on-premises SharePoint server (that is, a SharePoint server installed and run by the person or organization using the software rather than remotely). OneNote for Android currently does not support opening notebooks on such servers. However, you can open notebooks that are available on SharePoint Online.