Hi,
Setting "Add to cart button = Yes" is necessary, but it is not the only condition. HikaShop also hides the button automatically in a few cases, so check these on your course product:
1. Price. If the product has no price (no price row, or a price of 0), HikaShop treats it as free and hides the Add to cart button by default. Either set a price on the product, or, if the course really is free, turn on the "Display the add to cart button for free products" option in the configuration. Also, make sure the price is entered in the prices table, not in the "Retail price" input field which is for the MSRP.
2. Stock. If the product quantity is 0, the product is "out of stock" and the button is hidden. For a course, set the quantity to Unlimited (-1) or to a positive number.
3. Sale dates. If the product has a start date in the future or an end date in the past, it is treated as not on sale and the button is hidden. Leave both empty unless you need them.
4. The menu item. A "HikaShop Product Page" menu item has its own display options. Make sure its Add to cart option there is set to Yes (or to inherit), so it is not overriding the global setting with No.
5. Catalogue mode. In the HikaShop configuration, you have a "Catalogue mode" setting. The goal of this option is precisely to remove add to cart buttons everywhere, regardless of what is configured. So you need to make sure this setting is turned off.
In practice it is almost always the first point: add a price to the course (or enable the free-products option) and the native button appears. Once it shows, you can remove the Add to cart HTML you pasted into the description.
If you don't find the issue, please provide a screenshot of the settings of your product. The issue should be in there.