We Cannot Add out of Stock Item in Cart, So if we want to Add Out of Stock Item in Cart then we need to follow the Following Steps.
Search the item Globally which we want to add in cart if it is out of Stock
Click on edit.
Click on Web Store
Below Stock and Pricing Behavior
Choose the Out Of Stock Behavior
Allow back orders but display out-of-stock message.
write Out pf Stock message if necessary else leave blank
Click on Update the matrix
Click on Submit
Click on ok