Poor customer service

Good day

I am trying to do a purchase online, and in the process of linking my account, I get this messages:" The entered Account Number is linked to another user, please contact the Administrator."

I am the account holder and the account number provided is as on my monthly statement. Why do I get this message, is there another person linked to my account? This is a serious concern. I've dealt with Michellene and she doesn't deal with my complaint, she keeps on telling me my account is linked to my email, continuously. I am not able to purchase winter clothes for my kids as I am not able to access my own account.