MTX not working with fire spell on melee hit

Just got the elemental weakness spell skin to turn it green, using Talisman on weapon slot 1 and 2. Won't let me equip skin when it's socketed into my "fire spell on melee hit" from Smith of Kitava. It does let me equip on other cast on skills like cast on crit or ailment. I want my money back or fix this bug soon please.