Can anyone tell me how to keep an object hidden in Storyline when the player starts and it remains hidden till it is not triggered to appear?

Hi, Sugam

Entrance animations always play at the point the object starts on the timeline.

There are few ways to do what you want: 

• Put the sliding rectangle on a layer, and show the layer when the oval button is clicked. Note, though, that any objects on the layer will cover what's on the base. So you'd have to create a separate oval button on the layer if you want it to appear on top of the sliding rectangle. 

or

• Use a motion path to move the sliding rectangle from off the slide onto the slide. FYI: The default trigger for a motion path has it occur when the timeline starts. But you can edit the trigger to happen whenever you want. 

For more info about Motion Paths, check out Articulate's User Guide: 

https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-sync-motion-path-animations 

Perfect!! Thank you so much Judy Nollet. Highly appreciate your reply and sorry for the late response.

2 other solutions for this are...

Put a shape in the colour of your background above it to hide it.

or

use the build-in GSAP javascript to hide it at start.
var elementToHide = document.querySelector("[data-acc-text='ACC NAME']");
gsap.set( elementToHide, { autoAlpha:0 });

and offcourse if you want to show it then you can show it like this:
var elementToHide = document.querySelector("[data-acc-text='ACC NAME']");
gsap.to( elementToHide, {duration:2, autoAlpha:1 });