Thanks Sally for moving the thread.
I think I found the Problem but I dont know why my change is without effect. I check the Customizing of TRVFD and only find a field for extimated cost documentation. Not for the Input field.
So I look into the NetWeaver Portal role and open the page for creating a trip. The configuration for the state was set to 'required' by personalization. I set the state to 'not personalized' and save but without an effect. The Portal was restartet by the Basis and the state is still 'required'..