ykinfo: 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/ykinfo - from package: ykpers

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

On Debian possible default full paths are:

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

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

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