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The error "not a function" when using .map() in Reactjs typically means that you are trying to call the .map() method on something that is not an array or iterable, or that the array you are trying to iterate over does not contain a function that can be called by .map(). One common cause of this error is forgetting to initialize a variable or accidentally reassigning a variable to a non-iterable value. Another common cause is passing the wrong data type or an empty array to .map().