IMPOSSIBLE TO LINK SHOPPER CARD WITH ID

I want to use my Smart Shopper Card in the Checkers shopping APP, but the WEBSITE required in order to register the card says "That ID number is already registered, please use a different ID number"... YES, indeed, my ID number was registered by till - but how can it be possible upon opening your website's "Sign Up" page for making the card USABLE ONLINE, that the ID number is required, BUT that it is rejected? It is IMPOSSIBLE TO GET PAST SIGNING UP.