The term 'Sprite Sheet' in Phaser means a fixed-size sheet. Where every frame in the sheet is the exact same size, and you reference those frames using numbers, not frame names. This is not the same thing as a Texture Atlas, where the frames are packed in a way where they take up the least amount of space, and are referenced by their names, not numbers. Some articles and software use the term 'Sprite Sheet' to mean Texture Atlas, so please be aware of what sort of file you're actually trying to load.