Novak Djokovic can't enter the United States and he can't enter Canada either because both countries have the same country entry policy.
Novak Djokovic most likely will miss the US Open due to not being vaccinated as unvaccinated persons are not permitted to enter the country, at least foreigners. Djokovic confirmed at Wimbledon that he won't be getting vaccinated in any case confirming that he'll wait and see what happens next. Reports of a possible exemption have been shut down by the US Open so most likely nothing will happen.
Djokovic was also set to appear in Montreal at the National Bank Open however that won't be happening either as the Serbian can't enter Canada for the same reason. Aksed about it in a recent interview, Canada's Health Minister Jean-Yves Duclos said:
"Well first, the rules apply to everyone. There are a few exceptions but these are limited exceptions. So first of all, the rules apply to everyone, and second, COVID-19 is not over yet. And again, we need to be mindful of the fact that despite we have been so successful with two doses, we need to up our game and have more Canadians being up to date on their vaccination status."
Duclos did mention an exemption possibility which could still happen as Djokovic is technically on the event entry list but he would not comment any further. Whether Djokovic gets an exemption into Canada remains to be seen but it seems like his summer will mostly be a vacation.