The login forms on "my.ecwid.com" and on your store are different. They are for different types of accounts.
This is Ecwid control panel where you can manage the store, your products and settings. Only you as the store owner can log in there.
Your Ecwid storefront
It's a place where your customers can create their accounts, log in to view their orders and place new ones.
These are just different sets of accounts, that's why you cannot log in as your store's customer using your email from the control panel. If you want to test the 'Sign in' link in your storefront, you should register a customer's account (click on the 'Sign in' link and choose to create a new account).