When using Banktivity 8 or 9 the Cloud Document encryption password should be saved in Keychain Access on your Mac. Keychain Access is a utility application located inside the Utilities folder on your Mac, the Utilities folder is located inside the Applications folder – /Applications/Utilities/Keychain Access
From Keychain Access use the search field to search for:
Double-Click on the listing for ‘com.iggsoftware.service.sync’, then from the next window that appears, place a checkmark beside “Show Password”. Enter the Login/Admin password for your Mac (this should be the same as the password you use when your Mac starts up). Then you should see the encryption password.