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As the OpenCV's documentation for computeNMChannels says, it is:

    _channels   =   cv.text.computeNMChannels(  _src[, _channels[, _mode]]  )

so you have to specify the argument name like:

 channels = cv2.text.computeNMChannels(img, _mode = cv2.text.ERFILTER_NM_IHSGRAD)