help for downloading app on laptop

The error message is clear and reflects the system requirements:

Compatibility

macOS 10.13 or later, 64-bit processor

https://apps.apple.com/us/app/microsoft-onenote/id784801555?mt=12

To upgrade macOS read macOS - How to Upgrade - Apple.

What version operating system are you running?

brookefromnewton wrote:

Trying to download Microsoft OneNote app. I get an error stating we could not complete your purchase (null) can't be installed on "Macintosh HD" because OS X version 10.13 or later is required

What is the question?