[Help] fastboot error "Loading keystore failed status 5 [600] USB init ept @0x8f2270"

ok so i have a device, LG spirit H440n.
and i cant get it work.
if im trying to get into download mode, make a factory reset through the stock "recovery" mode or to boot normally i get this:

boot certification verify
Secure bootung Error!
Cause: boot certification verify.

i just cant do anything.
but i can enter fastboot mode. but when i enter i get the following:

fastboot_init()
Loading keystore failed status 5 [600] USB init ept @0x8f2270
udc_start()
-- reset --
-- portchange --
fastboot: processing commands

from there i cannot apply fastboot comands. if ill send "fastboot devices" i can see the device attached.
if i send "fastboot reboot" i dont get any error but device doesnt reboot, i just see in the device screen a new line "fastboot: reboot".
and if im trying to flash somthing i get an error "failed remote unknown command".
any solution?