Hi, if you are still having this issue with the Librem Key and Arch Linux, please install the AUR package: librem-key-udev this adds udev rules so that the Key is recognized in Arch Linux.
Also the Nitrokey app support for the Librem Key is still pending, it was already added, but will only be released in version 3.6.
You could compile the libnitrokey from source from the master branch or modify the AUR package for the libnitrokey library, that would bring the support for the Librem Key in the Nitrokey app
I was refering to the libnitrokey, the support for the Librem Key there has been merged into master. But still needs to be tagged and released as version 3.6