please see these detailled instructions here: Sen2cor 2.4.0 on SNAP 6.0
If you don’t want to use the SNAP gui, you can add the path of L2A_process to your system’s PATH (see here) and call it from the command line as described here: Sen2Cor-02.05.05-win64 - AttributeError: 'L2A_Tables' Object has no attribute '_L2A_Tile_PVI_File'