ykval-gen-clients: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/sbin/ykval-gen-clients - from package: yubikey-val

You may install the required package with command apt-get install {package}

Error in other languages:
ykval-gen-clients: bevel nie gevind nie
ykval-gen-clients: comandă negăsită
ykval-gen-clients: comando não encontrado
ykval-gen-clients: commande introuvable
ykval-gen-clients: command not found
ykval-gen-clients: käsku ei ole
ykval-gen-clients: không tìm thấy lệnh
ykval-gen-clients: komanda nerasta
ykval-gen-clients: Kommando nicht gefunden.
ykval-gen-clients: kommandot finns inte
ykval-gen-clients: komut yok
ykval-gen-clients: nie znaleziono polecenia
ykval-gen-clients: níor aimsíodh an t-ordú
ykval-gen-clients: no se encontró la orden
ykval-gen-clients: no s'ha trobat l'ordre
ykval-gen-clients: opdracht niet gevonden
ykval-gen-clients: parancs nem található
ykval-gen-clients: perintah tidak ditemukan
ykval-gen-clients: príkaz nenájdený
ykval-gen-clients: příkaz nenalezen
ykval-gen-clients: команда не найдена
ykval-gen-clients: командата не е открита
ykval-gen-clients: 命令找不到
ykval-gen-clients: コマンドが見つかりません