get_cat_ID doesn't return anything

I’m trying to use get_cat_ID to complete a url, but nothing happens… Where it says “Category” I put the name of the category to which I want to link, but when I pass the mouse over, I can see it takes the full path except the category number.

As far as I can see all syntax is OK, but like someone said, “all mistakes are human” I’m entitled to find out what’s wrong.

<a href="?cat=<?php get_cat_ID('Category'); ?>">ENTRAR | ENTER</a>

Does anyone know what may be happening?