Cola is a really hard flavor to nail. Pretty much any cola you buy in stores that isn't Pepsi or Coke is almost certainly going to taste like those cola-bottle gummy candies rather than the flavor you associate with Coke. There are a lot of different spices and citrus oils in there, and getting the proportion just right is really tough (it doesn't help that Coca-Cola is literally the only company in the country allowed to use coca leaf as a flavoring. They have a special permit from the FDA to import it.)

I almost always prefer drinking my own home-made soda to store-bought stuff, but I've given up completely on trying to formulate a recipe for a cola that tastes even remotely like people think cola should.

If you're deadset on making cola with a SodaStream, I'd go to a commercial restaurant distributor and get your hands on a box of real Coke or Diet Coke syrup.