Sinclair had proposed to acquire Tribune media about a year ago. Since the initial proposal, Sinclair has adjusted its deal to help win the approval of regulators. But it appears that it may not get approved after all, in fact, FCC's chairman Ajit Pai has filed a draft order that would send disputes over the stations that are owned by the two companies to an administrative law judge. Which could block the merger of the two companies.
The FCC Chairman stated in the order that he had "serious concerns" about Sinclair's buyout of Tribune Media and the selloffs of some stations. The current buyout would allow Sinclair to control the stations "in practice" which would violate the law, even though Sinclair's name wouldn't be attached to the stations officially. Sinclair has not yet commented on the FCC's move, but it is likely that Sinclair is talking with its lawyers to find out what it's next move is. Seeing as this is an ongoing acquisition, Sinclair likely won't be able to say anything concrete anyways. This acquisition could be blocked, as a similiar thing happened with Comcast when it attempted to buy Time Warner - which ended up being given to AT&T, who was also sued to have the acquisition blocked but the government failed to block it.
The reason for the FCC having serious concerns over this buyout is due to competition. Between Sinclair and Tribune Media, the two own the majority of local TV stations in the country, and combining the two would mean that more than half of the nation's local TV stations would be owned by one company. That's not good for competition. This may end up going to trial, but Sinclair will likely make some concessions to make sure that the FCC and other regulators will sign off on the deal. Which is a real possibility for both parties.