I cannot buy the Pro Version.
If I hit on the 9,99 Button, something flashes up for 1/2 Seconds and is gone again.
Reinstalled the app, Cleared all Caches, Repaired Store ....
OS + version: Windows 10 Professional
This really is a Microsoft Store / Windows bug that I have no power to fix. Make sure you have the latest Windows updates, no pending updates in the Microsoft Store app and try restarting Windows.
Try to reset the Microsoft Store cache as explained here.
If that all doesn't help, you can also purchase the Pro version here