ActiveStepIndex debe ser menor que WizardSteps.Count y ser al menos -1. Para pasos agregados dinámicamente, asegúrese de que se agregan antes del evento Page_PreInit o en el mismo.
Click to search this error in Google
English version
The ActiveStepIndex must be less than WizardSteps.Count and at least -1. For dynamically added steps, make sure they are added before or in Page_PreInit event.