In order to sign up and thereafter log in to your app.hellotax.com account, you need to use the "Login with Amazon" option in the app.
It is important to always use the primary Amazon email account (the original account setup email) when login in with Login by Amazon to your VATjar account. But how do we determine if the email address is the primary, sounds strange but many sellers setup their account years ago and have mutli-users?
Simply go to "Seller Central > Settings > USER Permissions"
Here you will see a list of current users, the top line is the primary user. *