Cannot join string values in python script

When I join plp_df string values with os.path.join, I get the following error. I can use + operation to join them.
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TypeError: join() argument must be str, bytes, or os.PathLike object, not 'Series'.
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According to the official PYTHON DEVELOPMENT GUIDE, Pipeline Pilot's String type is converted to Object in Python. Is it not possible to get the value as a pure string?