Access to block-system-main div
Looks like you're having a block of node teasers, and you want to initialize a slick slider on its items using vanilla JS, yes? There's a better way to do this, and that's to use the Slick module together with Slick Views.


Create a slick carousel view, configure it as a block and not a page.
Filter to the desired nodes
Configure to render the teaser displays
Configure the slider parameters.
On whatever block rendering mechanism you have (Block Layout, Context, Panels, whatever), position the view in the desired location. 


All that and you never have to touch code or meddle with the template.