Unity says that the private additive of a bool is incorrect in a groundcheck

I am just starting out using unity, and am trying to mess around with the movement mechanics to try and understand the way these processes work through experience, and while trying to use a tutorial to enable a groundcheck to limit jumping, unity has replied with the error message that the private additive to the bool of groundcheck is incorrect. Why is it incorrect and how would I go about fixing it?
(https://www.youtube.com/watch?v=c3iEl5AwUF8&t=822s) tutorial I used and I attempted to use the third method in the video (about 12 minutes in)

This is nothing to do with 2D-Physics or Unity, it’s C#. I’ll remove the 2D-Physics tag as that’s used when you’re asking about physics.

You are trying to define a method “IsGrounded” inside another method “FixedUpdate”. You need to define it at the class level like all the other methods.

Good luck.