With this article, you can know how to make the customer still buy the product even when product out of stock as this link demo: Demo
It’s used for the product which has 2 variants more.
Go to the setting product -> Click to “Edit”
On inventory, click on “Incoming”
Click to “Add a transfer” to set the product incoming
Input the quantity & Date
Finally, the text will show when the quantity of the product is 0