Check iCloud storage is enabled (on) for your account and your device.

iOS: go to Settings > Tap on your name > iCloud. Under Apps using iCloud, tap Show all and wait until your apps list. Find iReal Pro, Turn on.

 

Mac: click on System Settings (Preferences) > Click on your name > iCloud. Enable iCloud for your account.

System Settings (Preferences) > your name > iCloud > Apps using iCloud > click Show More Apps.

Find iReal Pro in the list of apps, Turn on.

 

iCloud sync will also be disabled if you have run out of  your iCloud storage.