In some cases, the game says you have more of a resource than you actually have available. This can potentially break the game because this allows you to put a MIRV launcher on a minigun.

Now you pass the condition check, just build it.

If success, the confirm will pop, the selection hovered on a different item. You can select an available mod/item, then press 'E' and quickly move the mouse to select another mod at the same time.