Many different PRT file types contain 3D parts or components, so there may be several different prt to ste conversions possible, unfortunately no way to convert prt to ste online seems to exist.
One of the prime examples for prt to ste conversion would be an early Solidworks (.prt) file to STEP product data file transfer (.ste). Unless you have access to SolidWorks, you will have to rely on some 3rd party CAD programs or dedicated converters.