Without the entire error log it’s hard to tell, but to me this looks rather like a problem trying to flash the app via DFU-util and not like a build error.
Hi @edi.auratus,
The screen shot seems to complain your device is not in DFU mode. Can you try to put it in DFU mode first manually? To confirm, you should see the LED blinking yellow.
I agree with @parhusip here. You don’t state what device you’re working with, but many ship with older firmware that won’t go into DFU mode automatically when flashing from Workbench. Put it into DFU mode manually and update DeviceOS before flashing your firmware.