What is the meaning of the disappeared ball which George W Bush wanted to catch?

The Oliver Stone Encyclopedia by James Michael Welsh and Donald M. Whaley provides this explanation:

Thereafter, the film ends with a final shot of the repeated dream sequence with W in center field. Reviewer Michael Betzold described the metaphor as follows: "Bush walks onto a baseball field in front of empty stands, imagines he hears cheers, and has the chance to make a big game-saving catch of a fly ball. But the ball never returns to earth," a "nice metaphor for Bush's simplistic incompetence."

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According to Wikipedia:

A fly ball or simply fly is a ball that is hit in the air, usually very high. Fielders attempt to catch fly balls on their descent.

Note that some reviews call the shot a "pop fly":

A pop fly or pop-up is a specific type of fly ball that goes very high while not traveling very far laterally. From the perspective of the fielder, pop-ups seem to come straight down. A fly ball is usually caught in flight and thus results in an out, called a fly out or a pop out as the case may be. Despite the subtle difference, however, the words fly ball and pop fly are often interchangeable.

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Thomas Pluck explains the metaphor some more:

A recurring image in the film is of George W. Bush alone in the outfield, the sun in his eyes, as a fly ball soars toward his glove. He was a son who grew up in a competitive and aristocratic family with a father who was both a war hero and a successful politician, when he only had a history of failures --- as an oil man, a financier, manager of a baseball team. When he got into Harvard and Yale, his mother congratulates him but his father sneers, “who do you think pulled the strings to get him in?” The pressure to show up Jeb, and even his own father was enormous -- and when he became President, it was something he could be easily talked into. In the beginning, I wanted to have a beer with W.; I voted for him. In the end, his personal failings -- his deep need to both impress and surpass his father -- led him to drag the country into a nation-building exercise in the Middle East at the cost of trillions, countless Iraqi lives, and over 4,000 American ones so far.

Robert Roten offers up a slightly different interpretation:

On the one hand, the movie is about a man who is the perfect example of the Peter Principle, a man who rapidly rises to the level of his incompetence (and beyond it). On the other hand, it is a story of a man who faces his demons and defeats them. In a poignant dream sequence early in the film, Bush finds himself in a baseball stadium, ready to catch a fly ball at the fence. He leaps and catches it, saving the country in his own mind. Near the end of his presidency, Bush dreams he's back in the ball park, he goes to the fence and looks up, but the ball has vanished. He looks for it everywhere. He can't make a play. He's run out of options and out of political capital. Game over.