If you have already created an account, and then cannot log back in, ensure that you are using the same login method as you used to create your account. Ie. if you created your account using Apple, Google or Facebook, you must use the same login method when re-signing into your account.
If you created your account using email, you can use the forgot password option to help you sign in.
NOTE: the forgot password function will not work if you created your account via Apple, Google, or Facebook.