Hello! Julianne from Mixpanel Support here.
Experiments are available on the most recent Growth plan. The most likely issue is that you're on an older version of the Growth plan. If you're on an older version of the plan, you would need to switch to the latest version to get access. You can see in the Plan Builder if you're on the latest plan, or feel free to write directly to Support, and we can check your plan from there!