Reflashing Fairphone 4 problem

After starting the unloched bootloader
“The boot loader is unlocked and software integrity …

PRESS POWER KEY TO CONTINUE”

comes a blank screen.
Certainly is my fault. I flashed an Ubuntu Touch whoch starts ok and I want to revert Android. I’ ve selected unmount partition first. The tool for flashing Android is not seeing the device even before I’v made that.
What Can I do?
Thank tou.