tag attributes

So basically I'm making a datapack where someone is randomly selected as a boss and fights everyone else, I created a tag that gets randomly given to a person then I'm trying to change that person's attributes using the tag that I gave the person but whenever I type @a[tag=juggernaut] (juggernaut is the tag) it gives me an error therefore I am forced to use @p which also doesn't work. Could someone explain how I could do this.