Error: flake does not provide attribute

I am trying to enter into nix shell but getting this error. Can anyone please help me. I am following Plutus Pioneer Program - Second cohort - Lecture #1 - YouTube tutorial. On 28 min of this video the instructor running nix-shell, So I have run that thing but got some error after that I read the plutus contributor guideline and run nix develop to enter into nix shell but got this error now.