PRT format is used for various 'CAD part' files from numerous tools, so it's always difficult to tell which particular format is needed. You have to pick software that uses or allows export to the particular type of .prt files you need. In case of 3mf to prt conversion, it's certainly related to export of 3D Manufacturing format.
Sometimes using dedicated 3D converters might be best, because they support many formats and you can simply select the one you need.
Updated: October 5, 2023