Three things:
The myth is around mostly thanks to astronaut Richard Gordon in the 1960s. Gordon claimed to have seen the Great Wall from space during his Apollo 12 mission, and his comments were widely reported in the media.
Carl, in the same book, In the book, Sagan mentiomns that the myth had been widely circulated for many years, but that it was not supported by scientific evidence. He went on to explain that the Great Wall is only a few meters wide and is made of the same material as the surrounding terrain, which makes it very difficult to see from space. Sagan emphasized the importance of being skeptical of claims that are not supported by evidence and urged readers to approach such claims with a critical eye.
The myth was long debunked before it was really brought to live by Gordon in the 60's. In 1932, the British astronomer C.T. Whitmell wrote in a book called "The Science of Space Travel" that the Great Wall was "impossible to be detected with the naked eye from the moon."