Custom JavaScript option is enabled from WordPress 4.9 and the higher versions. Besides, you need to make sure you have installed the Flexia Core plugin on your website. Read this documentation to learn how to install Fleixa core on your website. After installing this plugin you could find the custom JavaScript option by navigating to Customize → Custom JavaScripts from your WordPress dashboard. 

This is how easily you can find a custom JavaScript option on your WordPress website while using Flexia theme. Get stuck? Feel free to contact our dedicated support team.