On Soundiiz, you can create an account using credentials (login / password) but also by using Google, Twitter, Facebook and also Apple.
When creating a Soundiiz account with Apple, you will have the choice to share with us your true email or let Apple creating one for you. More information here: https://support.apple.com/HT210318
So, on our side we will get a random email address if you are not sharing your true email: https://support.apple.com/HT210425#hideemail
This email looks like "firstname.lastname@example.org". So once you have created an account Soundiiz with a "Sign in with Apple", you have to use it each time you want to connect to Soundiiz. The login/password method will not work here.
If you want to use then a login/password you will have to use Forgot Password on Soundiiz and use the email generated by Apple (you can found it on Soundiiz in your user settings). Or, in your Soundiiz user settings, change your email.