Shopify Variants have been edited?
Shopify variants being edited are the most common cause of the Add to Cart button not working.
It is important not to edit or rename the variants and prices in Shopify because they are managed by Kickflip. Modifying them in Shopify will not modify them in Kickflip and could break the connection between Kickflip and you cart.
What to do if you have edited your Shopify Variants?
If you remember what you have edited, you can set the values back to their original state and the problem should be fixed, otherwise :
Delete your Shopify Product
From the Kickflip dashboard product page, under Online Stores, next to your product, click on the more icon than click on Stop selling.
Click on + Add starting point, and select your starting point. From there a new product will be created in Shopify with the correct variants.