We are sorry to hear this!
Firstly, please double-check that you are inputting the correct email address that was used to place the order. If you are unsure, please feel free to contact our Customer Care team via our 'Contact Us' page and we should be able to confirm this for you.
If the email and order number is correct and you are still unable to access the return centre, please send over a screenshot of the error message you have received to our customer service team and they will be able to look into this further for you.