Google reCaptcha V2 problem

i always run into "You didn't pass the NoCaptcha verification"
in debug it says:
​

[google_nocaptcha7] => Array
        (
            [_error] => No secret key is provided.
            [var] => 
        )

​
but there is no possibility to enter the secret key in the design tab, there only the site key can be entered! where to enter the secret key???