Can someone please tell me why my battery want charge

You should always check the simple things first. Look for a bad battery cable or other electrical connection at the alternator. Also look for corrosion or glazing around the connections. The inside of a post battery cable can actually become glazed over enough to prevent voltage flow. The inner cable connections should be cleaned with a wire cable cleaning tool. I know that older beetles had voltage regulators I them, but I not sure about the new models.