If I play Bone Skewer can my opponent react with Not So Fast to it?

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

According to the card text for Not So Fast, it 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 you do not choose a target unit at that time. The selection of the unit happens during the resolution of the spell, when the opponent chooses a unit from their hand to play. 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.