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. OneNote for Android currently does not support opening notebooks on such servers. However, you can open notebooks that are available on SharePoint Online.