A 205/55/R16 tire is a 16 inch inner diameter tire intended for 16 inch wheels. If the current tires on your car are 225/45/R17 that means you have 17 inch wheels fitted and you cannot fit the new tires on smaller wheels.
The tire websites cannot tell the size of wheels you are currently using, as the manufacturer usually lists at least a couple of different supported sizes of wheels for each car model and a few sizes of tires for each of the wheel sizes. The website probably has a database of fitting wheel/tire combinations for most popular cars, but it's up to you to check if the tires you are buying will fit the wheels you currently own.
If the tire size is supported, you'll need new wheels to use these tires. You can always check the owner's manual to know what fits.