Apple has a special security setting: hiding the email address that is associated with the Apple ID and replacing it with another one (randomly generated).
This setting pops up for selection once per application.
If you choose to hide the originally used email address, it means a random email address will be generated every time you try to log in with your Apple ID.
However, ApprovalMax will recognise this as a completely new email address, so an attempt to log in with a random email address will be considered as the registration of a new user.
As such registration is not available in the mobile application, the user will consequently receive an error message and won't be able to log in.
In this case, in order to use Apple SSO, you need to change the Hide Email option in the settings of your device.
To do so, open your mobile phone's Settings -> Apple ID -> Password and security -> Applications that use Apple ID -> and find ApprovalMax, then select Do Not Use Apple ID.
Now, when logging into the application, select Show Email Address.