I do not see an option to sign up as an existing customer: Add this order to my account.
When signing up for an account, if you do not see the option for: Add this order to my account, check to see if you are using a coupon code. If you put in the hgc25 coupon code, the radio button will automatically be set to "I am a new customer" and you will not be able to change it.
I suggest you use the 20% off coupon, as it can be used by existing customers. The hgc25 coupon code is for new customers only.