I'm trying to remove the Armory research requirement for the Valkyrie but nothing happens when I click on remove item.

It appears impossible to remove the requirement once you've added it (or a preexisting one) - I've run into this myself. Guess the only thing you can do is replacing it with "Starport" - it will be superfluous but at least it won't get in the way.

Thanks, this works.

You need to click on the Buttons Sets tab before clicking on the Dat Requirements tab, because firegraft is a functional piece of software. Doing this should fix your issue.