You should take a look at the rank of the things you need to compound the War Halberd, then you would notice, that the Silvery Armor is rank 20 + Vertical Wing is rank 21 (I wouldn't try it with the Footman's Helmet rank 21, it takes to much money and material to compound it, and I barely have failures when making the Silvery Armor.). So you can't compound it IF you want to make it with these materials, with using Junior Alchemy, you need to compound it with Primary Alchemy.