Final answer:
Louis uses the excuse that gambling is going on at Rick's to close it down, although it's a pretext as gambling had been previously overlooked.
Step-by-step explanation:
The excuse that Louis uses to close down Rick's is that gambling is going on there. This is a made-up or "trumped up" excuse given the ulterior motives for the shutdown, as gambling was common knowledge and had previously been tolerated by the authorities.
Set against a backdrop of varied law enforcement standards during Prohibition and the overarching issues with organized crime and illicit activities like bootlegging and clandestine saloons, the closing of Rick's Café in the film 'Casablanca' touches on the themes of selective enforcement and corruption.
To answer your question, Louis used the excuse that D. gambling is going on at Rick's as a reason to close it down.