54mm is Breville's proprietary home-machine size (sold as Sage in the UK/Europe). Standard 58mm baskets and accessories do not fit these machines — you need parts made specifically for the 54mm format. Breville ships both single-wall (non-pressurized) and dual-wall (pressurized) baskets with most of these machines.
New to portafilter sizing? See espresso portafilter sizes explained for how 54mm compares to the other standards and why parts don’t interchange across sizes.