If you get the "SKU button is disabled" notification, this means that the AliExpress supplier disabled the variant your customer has ordered from your Shopify store. From the example below, you can see that the "United States" variant is disabled, therefore, Oberlo cannot select this variant and add the item to the shopping cart.
Suppliers block variants if they are out of stock or if they no longer sell them. Therefore, we recommend looking for another supplier on AliExpress who sells the same item and order it from them.
If you want to replace your current supplier, you need to use the "Override" feature. Oberlo will replace your current product and its variants with a link to the new supplier. Once complete, try to process your order again.
If you don't find another supplier, you will have to cancel your order and issue a refund to your customer.
Also, make sure your product inventory is updated automatically. Check out this article for further explanation.