Can't use skills with support

Hey everyone! I'm sorta new to poe and there's some stuff I don't understand.
For example. I'm using Sniper's Mark and want to combine it with Mark on Hit Suport.
However. If i do that it says "skill can't be used".
I've read it could be because of Mana issues but the skill costs 27 mana and with the multiplier from the support it would cost 54 mana. I have 732 mana.
Why can't I use it? I don't understand.

Even if I activate my auras I still have 327 mana left.

If something is set to be triggered you cannot manually cast it. If you hit a rare or unique enemy with an attack and have enough mana the mark will cast.

Oh! Thank's a lot! I get it now. That makes a lot more sense.