ykchalresp: 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 CentOS possible default full paths are:

/usr/bin/ykchalresp - from package: ykpers

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ykchalresp - from package: yubikey-personalization

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

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