Minimum supported OS versions for the app
Routespunkt Hiking Grid Diary v9.0.0 requires a minimum version of iOS 16.4 or iPadOS 16.4
v9.0.1 is compatible version of the app, for iOS 17 and iPadOS 17.
Check OS compatibility to your device, and update to the latest applicable:
The above pages have options to update to the latest version of the respective OS: