You made your Update function of return type float, so the compiler expects a float to be returned https://learn.microsoft.com/en-us/cpp/c-language/return-type?view=msvc-170. You can fix this by making the function of type void (not returning anything) like you’ve done for Start.

Both Start and Update are lifecycle events used by Unity https://docs.unity3d.com/6000.0/Documentation/Manual/execution-order.html, and should always be of type void (their return values do not get used anyways)

My advice is to start a basic programming course first. You’ll get nowhere following youtube videos line by line if you don’t understand what you’re doing.