FixedUpdate isnt working,My code isnt working

My code:
private void FixedUpdate()
isnt working!!
the error message says: A namespace cannot directly contain members such as fields or methods.
english is not my native language so i dont understand. help pls!!!,My code:
private void FixedUpdate()
isnt working. In the error message is says: “a namespace cannot directly contain members such as fields and methods”
english is not my native language so i have no idea on what this means.
Please help!!

Can you show the full code in your script?