Can you not cast so fast a Bone Skewer?

No, you cannot use Not So Fast to counter Bone Skewer.

According to RiftJudge FAQ #10218, Not So Fast can only counter an enemy spell or ability that "chooses a friendly unit or gear."

Bone Skewer does not target or choose any unit or gear when it is played. When you play Bone Skewer, you choose a battlefield, but the selection of a unit happens later, during the resolution of the spell, when the opponent chooses a unit from their hand. Because the spell does not choose a unit or gear at the time it is played (the moment it is placed on the chain and becomes a valid target for a counter), it does not meet the necessary criteria for Not So Fast.