I'm also had the same trouble. Old SD card doesn't help. I think the absent of Bootloader was main reason. I try to flash by STM32CubeProgrammer, but I catch stupid disadvantage - in some cases STM32CubeProgrammer can't connect to device in DFU mode. All drivers installed and function normally, Windows don't inform about any problem in Device manager. This is known problem with STM software.
In this case you can use ST-Link v2 to flash S6. SWD port is present, so flashing become simple procedure.