Raspi 4 with Noetic & 64bit compatible?

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I am trying to run Noetic on a 64bit RPi Ubuntu/Focal server version, but the following error does occur:

$ uname -a
Linux robot 5.4.0-1033-raspi #36-Ubuntu SMP PREEMPT Thu Mar 25 19:00:51 UTC 2021 aarch64 aarch64 aarch64 GNU/Linux

$ sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
Executing: /tmp/apt-key-gpghome.wGbRJBPKZs/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
gpg: keyserver receive failed: No keyserver available

Any idea why this is not working? On the installation page, the aarch64 architecture is only listed for arch/experimental. But I have read plenty of blogs which mentioned that this setup would work. Any hint? Thx a lot!