Google documentation states in multiple places that passkeys are supported in chrome Linux via google password manager. I haven't tried myself, are you saying that is incorrect?
https://issues.chromium.org/issues/417941069 more here.
For my excuse I should note, that this passkey support is relatively recent (end of 2024) and error message was really unhelpful to understand the issue. I was trying to use passkeys on my Linux computer like every few months since they started to appear on various websites and was frustrated by lack of support in Linux.
Thanks for pushing me into right direction, so at least I can now use them.