Why did Curtis say he had no money?

Like all gamblers, he knows that sometimes he wins and sometimes he loses. As the film goes on, we see how much of a problem Curtis has. He is very reckless and dejected. It's not inconceivable that he took a chance on the basketball match, not caring so much about the consequences.

Sure, he could have paid up. But he's also a high-risk drifter who will bet on absolutely anything, frequently crashing down with a hard thud.

This scene, juxtaposed with Gerry's scene trying to get in to the poker game, shows this nicely. They both gamble, with poor results.

Of course, despite this reasoning, it's clear many people weren't impressed with the scene. IndieWire, in their review of the film, said:

When Curtis bets on a random basketball game, and refuses to pay up, he winds up getting beaten and robbed. This scene and others like it have a random, aimless quality. The clichéd ending doesn't help...

...If Curtis doesn't mind letting Gerry burn all his money in gambling, why didn't he help pay off the loan sharks? The initial reason for their journey becomes less and less important to the character as the film moves along...

...Where are the consequences? When Gerry gets stabbed, he doesn't even need go to the hospital. It's impossible to sympathize with seemingly invincible characters.

This was one of many reviews I picked out showing complaints about the films arguably "meaningless" trials and tribulations.

So, whilst there is an arguable reason for Curtis' reasons - an out of control gambler with little concern for the consequences - there is also valid questioning of many of his scenes as simply not making sense in the context of his character and the film.

I agree that this sceen seems to make little sense. I was yelling at the screen as I was watching it (I was at home, thank you very much). Upon further reflection, however, I have concluded that the scene was intended to demonstrate the self-loathing that some degenerate gamblers feel. It usually stays buried deep, but sometimes an event will cause it to rise to the surface. In this case, Curtis was feeling especially bad about himself because he had deceived Gerry by betting on a different horse than they had agreed on and he had kept all the winnings for himself. He wanted/needed to be punished for this and this basketball game was the first opportunity that presented itself.

In the previous scene, Curtis wins the money from a horse race where he lied to Gerry about the winning horse. Knowing very well that Gerry is broke and has no where to go, he still cuts ties with Gerry. He feels guilty about making Gerry sell his car and all his belongings and wants to punish himself by getting to same Zero Gerry left with. He bets on basketball game expecting to get robbed of his winnings but the kids only take $100. He leaves the rest of the money for his mom later.