EliteCart warns you if a product is unavailable, so you can fix things right during the setup. This applies to
Addons
Gift-Rewards
Upsells
EliteCart then automatically
- hides unavailable addons
- hides unavailable rewards
- hides unavailable upsells
If a product shows as “unavailable”, it means something’s not quite right with how the product is configured in your Shopify admin.
General Product Status
On the product page in your Shopify Admin, check:
1) Is the product status active?
2) Is the product available in sales channel Online Store?
3) Is the product assigned to a Market?
Inventory Status
If above are fulfilled, check that the specific product variant has enough inventory? That means:
1) Either the variant is set to “Not Tracked”. If yes (see picture), it means that the variant can be sold 
2) If the inventory is set to track quantity then check whether inventory is available. For instance on the picture below we have four units available
3) Alternatively it is also okay if the inventory is set to continue selling when out of stock. For instance, in the picture below the product is available, too.
Market-Specific unavailability
If the product is indeed assigned to the market that you are shopping in (as we checked above), it could be the case that you have several inventory locations which are assigned to different markets. So the inventory of a specific variant might be sold out in the market your customer is shopping in.