When using ykman on Linux, you may find that the tool is sometimes unable to access your YubiKey for some of the commands. This is often due to USB device permissions, and can be tested by running the same ykman command as root.
The YubiKey is accessed in several different ways, depending on which command is invoked.
For smart card based applications, or when accessing a YubiKey over NFC, the access is done via pcscd, the PC/SC Smart Card Daemon. It’s usually enough to have pcscd installed and running for this to work.
Smart card access is required for the piv, oath, openpgp, and hsmauth
commands, as well as for any command issued over NFC.
The Yubico OTP application is accessed via the USB keyboard interface. Permission is typically granted using udev, via a rules file. You can find an example udev rules file which grants access to the keyboard interface here.
Keyboard access is required for the otp command.
The FIDO protocols are accessed via a USB HID interface. As with keyboard access, permission is granted through udev. You can find an example udev rules file which grants access to a large number (not just YubiKeys) of FIDO devices here.
FIDO access is required for the fido command.