Digital downloads in fedora music are hosted by the fine ecommerce platform payhip.com. When you select Digital Download to purchase a title, you should see a shopping cart:
There is a known issue with payhip's embedded shoping cart, which can fail to appear if you are connected to the internet over a VPN. Try suspending your VPN for a few minutes, refresh the page, then click the digital download button again. Alternatively go to Fedora Music on Payhip to browse the digital catalog and make your purchase.