when should i not meditate?

You should definitely not meditate during times when you should be absolutely focused on the task at hand, such as when driving.

While some forms of meditation may be fine to do certain times, others should absolutely not be done (you wouldn't sit crossed legged with eyes closed and meditate while driving, would you?).

Other than the obvious times to not meditate, there are also times when it may be more efficient to not meditate at, such as right before bed. If you experience tiredness to the point where you are having trouble staying awake, and it is interfering with your meditation, then that may just not be a good for you to meditate.
Same for meditation in the morning. If you wake up and meditate right away, but feel that you have not woken up fully, or enough, and are falling asleep during the meditation, then that is probably not the best time for you either.

Other than that, I believe it is up to you to find the best times to meditate at, and the best time to not meditate at. This is very individual and there is no rule that applies to everyone.

Good luck!