Fairphone 4 - Unlock Bootloader failed

Hello everyone, I hope someone can help me.

I want to unlock the bootloader, but I can’t get along with the instructions.
Of course I’ve done developer mode, OEM unlocking and USB debugging. Fastboot is installed.
I am following this guide: https://support.fairphone.com/hc/en-us/articles/4405858258961

So I enter “ADB reboot bootloader” and the smartphone goes into fastboot mode.
Now I enter “fastboot flashing unlock”, nothing happens on the smartphone, ADB shows me “Waiting for Device”
According to the guide, I should now follow the instructions, but apparently I have to do something else first? The only choices are “Restart bootloader”, “Recovery Mode” or “Power off”…

Can someone tell me what I have to do? Thank you very much!
Many greetings!

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