add SSH key after provisioning to production device?

@Hades32 I got WSL2 up and balena-cli running in it, see How-To: How install balena-cli in WSL2 if behind Proxy like Zscaler - Product support / balenaOS - balenaForums.
I tried now balena-io-playground/ssh-key-insert: Tooling to insert the relevant SSH keys into balena devices’ configuration. (github.com) but I get

:~/balena.io/ssh-key-insert-master$ bash -x ./run.sh
+ cat batch
+ stdbuf -oL xargs '-I{}' -P 10 /bin/sh -c 'grep -a -q '\''{} : DONE'\'' sshkey.log || (cat add-ssh-key.sh | balena ssh {} | sed '\''s/^/{} : /'\'' | tee --append sshkey.log)'
<usedr-name>@ssh.balena-devices.com: Permission denied (publickey).
Warning: ssh process exited with non-zero code "255"

Are the SSH keys correctly configured in balenaCloud? See:
https://www.balena.io/docs/learn/manage/ssh-access/#add-an-ssh-key-to-balenacloud
Are you accidentally using `sudo`?

I’m still clueless about which key is expected for what action and how keys are distributed…?