Of course Donald Trump can accept defeat because in his mind, he can do anything. He will fight harder than hard for victory but if he should be defeated, he will do whatever is necessary to spin that into a positive light. He will actually find a way to speak of his own defeat so that it appears to be a victory.
Trump doesn't accept reality, Trump tries to create his own reality. He proves this every time he speaks, as he's always trying to create some fictional narrative that doesn't really stand up to closer scrutiny. This worked for him during the GOP primary, in which voters were a little more easily led around by the nose. However, during a general election, more people are paying attention and won't stand for this attitude. Trump will be sure to blame the system, blame the media, and blame "Crooked Hillary." He'll blame anything but his own incompetent campaign.
Donald Trump is slipping even further in the polls to Hillary Clinton and this is not even a surprise to his supporters. His own party pulled out from underneath him at the convention and now the "Trump fans" are going back to being silent racists who mutter under their breath. Trump will make a great scene after November.
Donald Trump will not accept his defeat graciously. He is already talking about the "rigged" system, and how the election will be stolen from him. This comes about despite his offering no proof of any such conspiracy. In his world, losing the general election would equate him to being a loser, which he cannot accept.