Disable : Journal > Product Extras > Extra Button for Notify Stock (if added)
Enable : Journal > Styles > Product Page > Add to Cart > Out of stock visibility > Enable
If you see the notify button is not aligned properly on product pages, try the following
Add the following style to the class .notify-button
width:50%;
For OpenCart 2.3.x.x and Journal3 Users
Go to Admin : Extensions > Extensions > Select Themes from drop down > Edit Default Store Theme. Choose journal3 for theme directory.