The most probable answer is that the email address you are entering has not been provided. If you registered to us by telephone, either recently or historically (you only need to have ever called us once in the past), then your email address would not have been captured. If you did register by making payment by phone and it is your first time of logging in to myRingGo you should enter the mobile number.
Another plausible example of this happening is if you are a business with multiple employees registered against different mobile numbers and using a personal account rather than our Corporate offering. An email address can only be stored against one account; meaning the email address and PIN combination you are entering is likely linked to a different mobile account than that you are attempting to access. You should instead enter the mobile number and PIN rather than email address in these situations.