I'm not sure what you mean.

Did you book a reservation? If so, via which channel and for which train (route, date, departure time)?

Or did you add a journey to My Trip or My Pass? That is not booking. You can delete journeys whenever you please. You can also cancel future travel days (including inbound/outbound journeys) or cancel the activation of your pass if the start date is in the future.

If you want to cancel a travel day that has already started, then only customer service can help you:

https://eurail.zendesk.com/hc/en-001/requests/new

Clearly describe the problem and the steps you did.

@Nicole Halbauer 

Du kannst auf Deutsch schreiben wenn das einfacher ist.