No reliable sources indicate that an American flag was found under a man's feet in Afghanistan.
On August 15, an organization known as Russian Top (Topru.org) uploaded a photo of men wiping their feet on the American flag. The Russian website states the militants effectively dismantled the pro-American regime and removed everything associated to America from Afghanistan between August 6 to 15. On Twitter, the exact photo was circulated, claiming to show two Afghan citizens.
The Hill reports the U.S. Embassy in Kabul, Afghanistan, took down the American flag after the Taliban controlled the city. CNN first reported that the lowering of the flag was the penultimate stage in the evacuation of American workers from the embassy.
Although some images appear to show an American flag under the feet of certain men, no evidence has been found in the mainstream media or from reliable sources. It's also unconfirmed whether the individuals were Taliban members or not or whether the image was taken in Afghanistan. As a result, the claim is categorized as unverifiable.