Modal Popup of Avada is based on Bootstrap so any plugin using the same bootstrap modal will conflict. The common plugins that conflict are Bookly, WordPress Download Manager. In order to resolve this issue, you can dequeue the bootstrap modal script of Avada theme. Below PHP snippet can be inserted in functions.php file of your Avada Child Theme and make sure to activate your Child Theme. This way the conflict will be resolved and you’ll be able to use the Modal Popups of Avada and your third plugin won’t need to be disabled anymore.
Hopefully the above PHP code to dequeue the script of modal will resolve the issue.