- Are you uploading via USB-to-UART converter connected to the board? If no, the upload protocol is wrong, chose the fitting one.
- If yes, to which pins did you connect the USB-to-UART converter? See https://www.st.com/resource/en/application_note/an2606-stm32-microcontroller-system-memory-boot-mode-stmicroelectronics.pdf page 141
- Was the chip booted with BOOT0 = HIGH, BOOT1 = LOW?