Novak Djokovic can't compete at the US Open as of now because the Serbian has no legal way of entering the United States.
Obviously, it's because of his vaccination status which prevents him from entering legally. Djokovic refused to get vaccinated which bars him from entry into the US as a foreign national but he could get in via an exemption. So far no indication has been given that he might receive one with even his coach Ivanisevic admitting that it probably won't happen.
Djokovic is a bit more optimistic training hard in case he does get in and many are urging behind the scenes. One of them is John Isner who publicly confirmed he is trying to persuade authorities to allow him to enter. He did via. Twitter while responding to a question about whether some US players are lobbying on Djokovic's behalf.
He responded:
"Yes. Completel lunacy he can't compete as of now."
He mockingly added a #science hashtag to end it.