Please note that in the command you ran, ‘_U’ is a typo, it should be ‘-U’ — i.e. a dash, not a underline.
Also, what follows that is NOT ‘flash:wos_219.hex’, it should be ‘flash:w:os_219.hex’. It’s better that you copy-paste the command from the instructions:
https://openthings.freshdesk.com/support/solutions/articles/5000832311-opensprinkler-2-x-firmware-update-guide
which will reduce the chance of error in typing the command.