Are you sure you've previously registered an account? We do not require you to register an account to use Paprika so you might not necessarily have one. Instead, by default, your recipes are stored directly on your device. You can optionally go to Paprika's Settings -> Cloud Sync and register an account, which will enable your recipes to be automatically backed up to our cloud servers.
But if you never actually registered an account, and then you try to login to Paprika on another device it will not recognize your email because you don't actually have an account. You can check whether you have an account by going to Paprika's Settings -> Cloud Sync on your current device. If you have an account set up it will show you the email address of the account.
If, instead, it shows a sign up screen, that means you were using the app without an account, and you should instead press Sign Up to create one. (We generally recommend creating an export of your recipes in the Paprika recipe format by going to Settings -> Export Recipes first, for backup purposes before signing up for a new account.)