The error message is letting you know that the UnityEngine and System.Diagnostics namespaces both contain a Debug class, and so the script can’t possibly know which one you want to use unless you specify it (by typing either UnityEngine.Debug or System.Diagnostics.Debug).
In your case, just change the Debug line to:
UnityEngine.Debug.Log("The GameObject with the tag 'dk' has been collided with.");
Alternatively, you can just get rid of the System.Diagnostics line at the top unless there’s a specific reason you need it.
Also, please use code tags when posting code in the future!