Google Maps shows "For development purposes only"
16:18 21 Jun 2018

Google Maps show the message "For development purposes only" when I try to show it in my webpage:

enter image description here

How could I make this message go away?

My code is like that:

Then later I have 


  Route berechnen

I don't see where this message originates from.

html google-maps google-maps-api-3