C# Error with random number generator. "Error code : CS1061. Doesn't contain a definition for 'Next'"

My number guessing game random number generator doesn't work correctly. Script : https://gist.github.com/Vlatkoski/694796a430084f0f1b8bc888233be5fa

Screenshot : see picture(click here)

Expected: Run without errors and retuen a random value between 0 & 1000.