I believe I've found a bug concerning the multi-weapons and their potentials as I've had several others via ship4 confirm it with me as well..I personally have 2 +40'd multi-weapons ( vialto spear-tali and glissen launcher-tali). Both have Lv2 potentials and I have both the coin and mats to do the upgrades however when it reaches the "confirm" screen "yes" is no longer highlighted as a selectable option..not sure if this is some kind of bug since patch or if it was somehow intended to be intentional but have had several others via ship4 say they've seen this issue as well so thought it worth bringing attention to.