If I play Carnivorous Snapvine and have to resolve its effect, can I choose not to kill the enemy unit at the battlefield?

No, you cannot choose not to resolve the effect if there is a valid target.

According to the rules and FAQs regarding Carnivorous Snapvine:

• Mandatory Ability: The ability "When you play me, choose an enemy unit at a battlefield. We deal damage equal to our Mights to each other" is mandatory if there is at least one valid target on the battlefield.
• Targeting: You must choose an enemy unit at a battlefield if one is available. If there are no enemy units at any battlefield, the ability cannot be triggered or will fail to find a target.

As noted in the FAQ: "The ability is mandatory if there is at least one valid target." If you play the card and an enemy unit is present at a battlefield, you are required to choose it and proceed with the damage exchange.