To resolve this issue please update Quick Sale to version 2.8.8 or newer.
To update the app, open the App Store and search for Quick Sale - Pro Invoicing, an Update button should be available. You can also update from the Updates section of your App Store Account panel (App Store > [Your Account Icon - upper top right]).
If the issue continues after updating the app, please restore the purchase by following these steps:
1) Open Quick Sale and go to Settings
2) Tap Subscription
3) Tap Restore Previous Purchase.
* Important: This issue was fixed in version 2.8. Please update to the latest version if you have this problem.
The Calendar dates may not be visible on iPadOS 15 in certain circumstances. A fix for this issue will be available soon.
In the meantime, to be able to select a date from the calendar, with the calendar open, move or rotate the iPad. This should make the calendar appear.
If this doesn't work, please make sure "Rotation Lock" is disabled on your iPad.