Test pad D7 to GND. If the device is missing the bootloader D7 will be held high with a weak pull-up for JTAG/SWD.
If you had a blue LED on D7 like the Electron, it would be on, dimly.
If the problem is indeed the missing bootloader, then you’d need to flash that using SWD using a ST-LINK/v2 JTAG/SWD programmer (or similar). It needs to connect to D7, D6, 3V3, and GND.