Dear gentleman,
it seems you are trying to connect via UART/RS232 boot loader , in that case check that the boot mode is well selected using BOOT(0/1) pins in your hardware and reset the MCU.
if you have JTAG or SWD , you need to change the software configuration of the STM32CubeProgrammer yo that mode,
Hope it helps,
Cheers,
STOne-32
