Is there a monument on the Great Wall that says "You are not a true man until you reach the Great Wall"? If so, where is it?

Yes, at the foot of the Badaling Great Wall, on the 858-meter heroic slope, there is a famous saying that "you are not a hero until you reach the Great Wall".

"He who doesn't reach the Great Wall is not a hero" comes from Qingpingle Liupanshan written by Mao Zedong in June 1935+00: the sky is high and the clouds are light, and you can see the flying geese in the south. If you don't reach the Great Wall, you are not a hero, but you will fight for 20,000. At the top of Liupan Mountain, the red flag flutters in the west wind. Holding a long tassel today, when will it be a black dragon?

This poem was written when Chairman Mao led the Red Army across Liupan Mountain. It vividly shows Chairman Mao and his Red Army looking up at the sky after successfully climbing Liupan Mountain, and expresses their firm determination to completely defeat the Kuomintang reactionaries and Japanese imperialist aggressors, as well as their lofty desire to carry the revolution through to the end.