​@sakaeru I recommend filing a Support ticket as described here they may be better able to determine whether a WooCommerce API call from Klaviyo was failing 

Hi ​@sakaeru,

Has this issue been resolved for you yet? If not, there are a couple reasons why this might be happening. Firstly, have you created the coupons in WooCommerce? The coupon needs to be created in WooCommerce before it’s created in Klaviyo. Otherwise, it’s expected that you’ll see an error like that. Our guide here explains how to create these coupons in WooCommerce.

After that, can you check and make sure the store URL on your WooCommerce integration page (in Klaviyo) is your actual store URL? We’ve seen instances where customers changed their store URL, and the connection between Klaviyo and WooCommerce experienced errors after that. If your URL doesn’t match the URL in Klaviyo, you’ll want to remove the WooCommerce integration entirely, then re-integrate.

One more reason for this issue could be due to the minimum inventory of the coupon. When you create a coupon in Klaviyo, these are generated in batches according to whatever the minimum inventory is set to (if minimum inventory is 5, then every day at least 5 new codes are generated). Your minimum inventory value should always be set to the anticipated number of daily sends that contain the coupon. If your minimum inventory is too high for your WooCommerce server to handle, it’ll error out, and no coupons will be created. It might be worth trying to lower the minimum inventory to see if this allows coupon creation to happen.

Let me know if the above helps, or if I can answer any more questions!