[justy]

I think you're just missing an {else} and a closing {/if}:

{if $cart.coupon_discount eq 0 and $products ne "" and $membership_levels ne "Wholesale"}
{if $active_modules.Discount_Coupons ne "" }
{include file="modules/Discount_Coupons/add_coupon.tpl"}
{/if}
{else} # You need to provide an alternative if the conditions fail.
{/if}