Has it ever worked on this board? Assume we know nothing about the specifics of your implementation.
Use the STM32 Cube Programmer Tool when doing initial tests.
Check SWDIO, SWCLK, NRST, VTARGET and Ground connectivity. If the part doesn't start/function you aren't going to be able to connect to it.
Check supplies, and VCAP voltages and capacitors.
Check the state of NRST
Try with BOOT0 pulled HIGH
