This is an issue Rick covers in a later lecture, but it’s to do with both the UnityEngine and System namespaces having a Debug method, and the engine not being able to determine which to use.
In your statement you could add in UnityEngine.Debug.Log() and see if it fixes the error, or you could remove the
using System.Diagnostics
line from the script (as I dont think they’re being used this early on in the course)
I dont recall having this issue in my project but like I said Rick does cover this later on and explains exactly what’s going on with Namepsaces