No one knows. Venus as a wandering star was known to the ancient Egyptians and Babylonians based on their writings and tables, but we do not know the name of the individual who discovered it. The earliest cuneiform inscriptions about the Star of Ishtar come from the Venus Tablets of Ammizaduga, which were produced by the Babylonians around 1650 BC. Ancient Egyptians also knew about Venus, which they called Hathor's Star and often associated it with Osiris. But their mentions of Venus seem to appear much later in their history during the Greek Ptolemaic era. We have the same problem with the Sun and Moon. We don't know who 'discovered' them either.