eBay gift cards have conditions and limits and can not be used to purchase all items.
https://pages.ebay.com/giftcard/terms_conditions.html
Some terms that might be relevant:
If none of that applies in your case, try splitting up your purchase or purchasing your items individually to see which items might be preventing the combined order from going through.
Once you have used the gift card once, any remaining balance should appear as a line item on the checkout page with a check box that can be applied to a future order.
