I read Marco Polo never wrote about the Great Wall of China which some people say is proof that he never actually went. Is there any proof to this claim?

Marco Polo's travels took place between 1271 and 1295, reaching Beijing between 1271 and 1275.

The first part of the modern Great Wall of China, as can be seen today, was completed in 1474. While there were previous Great Walls, these were actually principally constructed from compacted soil, and in areas where there was human habitation, this soil would often be taken from the wall for spreading on fields or for creating housing.

Other European visitors to the Far East during the Middle Ages, John of Plano Carpini, William of Rubruck, Giovanni Marignolli, and Odoric of Pordenone, all also failed to mention any wall, even those who went to Beijing or even crossed the line of the old walls.

Sources:

Arthur Waldron: The Great Wall of China: From History to Myth

Stephen G. Haw: Marco Polo's China: A Venetian in the Realm of Khubilai Khan

EDIT: Just remembered another anecdote: Ibn Battuta, a Moroccan explorer visited Beijing in the 14th century and inquired about the wall created to contain Gog and Magog, as mentioned in the Koran. Despite asking in China no one connected his question to a great wall.