Make the Sacred Band cheap, easy and quick to train, if you desperatly want carthage to build them. There is no way afaik to directly force a specific unit's recruitment. Also, if the lack of soldiers happens because of lack of money, you can always give Carthage a financial boost. btw, AI will recruit soldiers that will also be effective against the ones it's facing, ie it will train spearmen if it faces lots of cavalry units etc.