Bookers see a "Ouch! We can't show you this booking page" error

If you see this message on your live booking page: 

That means there are paid features enabled on your free account, or that you are linking to more calendars than you're paying for on a paid account. 

To resolve this message, upgrade your account for the correct amount of calendars, or for free accounts, visit your booking page settings to turn off any feature marked Upgrade.