You've got the order of everything backwards. First you need to set the "Caster" I.e. triggering unit, then the location, then the group, then you'll loop through the unit group.
Then you should also do the change as the previous reply mentioned, 'picked unit'.
But you also need to change the "matching (((Player 1(red) controller) equal to player 1(red) blabla) to "owner of picked unit = owner of Caster" or "owner of picked unit is an ally of owner of Caster = True"
And finally, simply adding 5 armor won't work since that will be permanent unless you remove it with a trigger again.
Why are you using thunderclap, does the ability deal damage? If not, I would suggest using battle roar or something as that's an AoE ability that will allow you to give units armor temporarily without any trigger.
If you're using thunderclap for a certain reason or whatever, it will get a bit more complicated. Still, the easiest way to solve it should be to make use of a "dummy" unit, you simply spawn an invisible unit that will cast the battle roar spell.
p.s. you should also change the event from "beginning to cast an ability" to "starting the effect of an ability", otherwise players will be able to abuse this spell by cancelling it before it's fully cast, thus it won't cost any mana or have a cooldown, but you can still get the trigger effects. (So basically infinite free healing.)