Variable not in scope

For every piece of code that I wrote, I get ''variable not in scope''. (for example :
length1 :: [a] -> Int
length1 l | l==[] =0
| otherwise = 1 + length1 tail l
some of the other solutions i tried are written by my teaher.

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@ThatSmart It started working..?! I didnt change anything about the code I just clicked the run button first and did exactly like in the screenshot I sent. Thats so weird. Thanks for the help.