Log In With Apple for the Mac OS App
When I visit the Yousician website, Log In With Apple is available and works great. When I use my iPad or iPhone, same thing - Log In With Apple works great. When I download the app to my Mac, the option is absent, so I am not able to sign in on the MacOS version of the application. Please enable Log In With Apple on the Yousician Mac OS App.

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Daniel Plummer commented
So you can get around this by logging in via Apple on the website and then going to your account options. This will give you the email address being used (which might not be the same as your Apple ID if you are masking it) and allow you to create a password for Yousician. You can then use this to login to the mac app.
This shouldn't be necessary though - don't understand why there is not simply a login with apple option on the mac app.