flashing pico with stm32loader.py

You probably want to erase first as well? see http://www.espruino.com/Serial+Bootloader

python stm32loader.py -p /dev/cu.usbmodem1421 -evw espruino_1v87_espruino_1r3.bin 

Hopefully on the Pico you don't need -k - not that it'll do any harm.

If this a binary that you built yourself? If so, you need to offset the address as well with -a 0x8004000, since the Espruino binary sits on top of the bootloader in memory (images from the Espruino site contain the bootloader as well)