The button Registration shows the Registration_Overview for certain TrainingEvent. That is the reason of the error-message: without knowing for which TrainingEvent it needs to show the registrations, it won’t be able to show them. Therefore the button triggering this Registration_Overview, needs to be located inside a Dataview or listview of a TrainingEvent. That is no so in your current setup.
You can either move the button to a TrainingEvent’s Dataview or Listview.
Or you can make the button trigger a TrainingEvent-select page, which in turn show the Registration_Overview.