MTG Salvation

I was just playing an MTG Arenas game and had dreadhorde out attacking and lava coil in my graveyard. my plan was to attack with dreadhorde and use lava coil on a blocker. however when it gave me the option to choose instants or sorceries from my graveyard lava coil could not be chosen. I don't understand why because to me it seems to meet all of the requirements. it was in my graveyard dreadhorde was attacking and lava coil is equal to the mana cost dreadhorde is. I'm new to magic and just slightly upset as to why I couldn't use it. If anyone could answer this for me i'd really appreciate it thanks.