How can I activate the JS tab? 

I do not have a JS tab:

The guide is for the pages app ( ACP => Pages => Templates ) To customize the existing IPS javascript code, you'll need a custom application or plugin

https://invisioncommunity.com/developers/docs/development/applications/structuring-your-application-r111/

https://invisioncommunity.com/4guides/themes-and-customizations/javascript-framework/introduction-to-the-framework/mixins-r285/

Your screenshot is from the main theme - you cannot edit the JS of the main theme from the ACP.

The linked article is a guide on how to modify Pages functionality, where you can modify the HTML/CSS/JS of Pages itself.

@Daniel F and @Nathan Explosion, thank you. Now I understand. Topic is closed.