There are sometimes some problems with some Apple IDs which can be solved by signing out and in with your Apple ID on your iOS device.
- Open the app store app on your iOS device
- Switch to the "Highlights" section and scroll all the way down
- Tap once on your Apple ID and choose "Sign out"
- Open WorkTimes and try to purchase again
If you still receive an error please contact me at firstname.lastname@example.org.