Can you cast Not So Fast on Cull the Weak?

No, you cannot use Not So Fast to counter Cull the Weak.

According to the card text and official rulings:

• Not So Fast can only counter a spell or ability that "chooses a friendly unit or gear."
• Cull the Weak does not target or choose any units when it is played. The instruction to kill a unit is performed by each player during the resolution of the spell, rather than being a targeted choice made when the spell is played.

Because Cull the Weak does not meet the requirement of choosing a friendly unit or gear, it is not a valid target for Not So Fast.