If your add-on is unavailable that ultimately means that the Shopify product variant that you selected is not available or sold out in the specific market your are viewing in your store.
EliteCart automatically hides addons if the respective product is not available in the market your customer is shopping in.
For more information please view the following guide:
Why is my product “Unavailable”?
Learn how to fix an unavailable product for your addons, rewards or upsells.
Also ensure that the addon is not hidden due to EliteCart’s Shopify Market settings or Schedule settings.
Schedule Settings
Control when cart add-ons and banners appear based on your store's timezone