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You can register up to 3 registration numbers for your parking permit.
See manual here

In 90% of cases this is because you have not approved your card for online payments. Log in to your bank and allow the card to be used for online payments.

First and foremost, check your spam folder in your email account and make sure the email has not ended up there. If you still cannot find any email even though you are sure that you entered the correct address, then try creating a new account with a different email address.