If your Mixpanel connection failed this is likely due to your Mixpanel subscription; specifically if you are using the free version of Mixpanel. Mixpanel does not offer API support for their free plan for us to grab data.
The other most common reason that the Mixpanel connection could fail is that only the US version of the Mixpanel API is currently supported in Visible.
If you have a different Mixpanel plan and are still having issues please send us an email at support@visible.vc