If you have used the 'Sign in with Apple' feature, you may be given a username and email address that you do not recognise. This is because you decided to hide your Apple ID email when signing up with Memrise. You can read more about this at this link.
If you are already logged into your app, you can change your email and username in your app settings. You can then use these new details to log into any platform.
If you’ve shared a private relay email address with Memrise, you can also view it and manage it in your account settings at Settings > Apple ID [Your Name] > Password & Security > Apps Using Your Apple ID.