Microsoft has decided to no longer offer the Windows 10 Notepad through the Microsoft Store. Microsoft announced in August that they would be pulling it out of the normal Windows release cycle and move it to the store where they can offer more frequent updates. This will allow them the flexibility to respond to issues and feedback outside of the bounds of Windows releases. The latest Windows 10 Insider build will uninstall the Notepad from the store and replace it with the version being distributed with the build. This also means that Insiders will have to wait for new Notepad versions to be released as Microsoft releases new Windows 10 builds.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-notepad-on-microsoft-store-is-no-longer-supported/