Best advice is to learn the basics of C# programming in Unity and the basics of the UI system.
With that, you’ll be able to put together something like a cooldown timer.
Check out the learn section & subsections here on the site: Learn (navigate around a bit for subsections).
If you want help with a script, post what you’ve done & what you want with the script, including posting it in the thread: Using code tags properly
I don’t know what “unable to edit the script after the image” means.
Hope that helps at least a little bit.
You want to:
a) throw the grenade, begin the countdown.
b) prevent further grenades, until the timer expires
c) re-enable the ability to throw when the timer is over
d) include this info/updated (time) in the UI