I found my error finally. In the Sub OpenAI_TextToSpeech_Bot, the last line OpenAI_TTSResponse(savePath, "tts.mp3") was not supposed to be there.
Note that in the attached code, the OpenAI library is modified to allow the user to select the path were the mp3 file gets saved.
Since I don't have permission to update the OpenAI library and don't know how myself, this is it.