Make sure you have not deleted the product and are using the latest button code from Seller Admin for your product.
If you are not using our full button code in your sales page but, rather, using a link-maker widget in your page editor or sending/posting a bare link URL, this help page explains how to obtain the correct URL from the full button code we provide.
You might have pasted the Add to Cart button code before customizing the product's Item Number in your Seller Admin, so the original Item Number referenced in that old button code would no longer match the current Item Number in that product's settings. To fix this, whenever you change a product's Item Number, be sure to copy the latest purchase button code we provide for the product to paste into your site, or you can just edit your site's existing code to reference the new Item Number (the
&i=XXXXXX component of your button URL, where
XXXXXX should be the product's current Item Number).
Also, if your product is using Variants which affect price/weight/stock/SKU, that error can appear if you select a combination of options to Add to Cart when that particular combination is missing from your Variants configuration lines.