Why is a button grayed out and I cannot click it?

A "gray" button means it cannot be clicked and executed. Usually, this means you are not meeting the conditions for it to work. For example, if you select more than two layers, the "Slice" button will remain gray and not be available to click.

So the first step to solving this problem is to make sure you're meeting all of the conditions for a button's function to work. Usually, that means having the proper number of layers selected.

But when you feel confident you are meeting the conditions and the button is STILL not available, sometimes it's just that the application is confused. In those cases, I recommend saving your work, restarting the application, and re-opening it. This often clears up the issue and allows you to proceed!