Most likely, the gift was marked as purchased on your reservation page, which removes the ability to buy it. Return to your reservation page with Babylist. You can get there by clicking on the link that we sent you over email with the subject line "Your gifts for registrant's name (and what to do next)".
From there, un-check "I purchased this". This will bring up the red button(s) for each item. Click those to be re-directed to the retailer where you can purchase your gift.