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The "XXX is not defined" error is a common JavaScript error that occurs when the code is trying to reference a variable, function, or object that has not been defined or is not in scope. This error can occur in React when a script that references an undefined variable or function is added to the public/index.html file. The solution to this error is to ensure that all required variables, functions, and objects are properly defined and imported before they are used in the code.