Legend affix for weapon damage and elemental damage % cannot be rolled. Use a base item that has it and add other affixes. Also you can't stack the same legendary and epic affix on the same item so don't bother trying to add epic weapon damage % to a weapon that has the legendary affix already. You can add a mythstone after for affixes available by mythstone to try to double up.
Also elemental damage is named different based on the currently element type of the base item. Use the lowest level crystal to roll it to your preferred element first if you are not familiar with the naming conventions of the 5 different elemental variations.