Can't find Microsoft One note in iPad's appstore
What’s going on is your iPad doesn’t meet the minimum version, so the listing is likely suppressed. It also could be a country region restriction, but that’s harder for others to test for you. The simple reason is you didn’t purchase an older version of the app to re-download that and/or the developer removed the old version entirely.

https://apps.apple.com/us/app/microsoft-onenote/id410395246