ykksm-gen-keys: 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/ykksm-gen-keys - from package: yubikey-ksm

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

On Debian possible default full paths are:

/usr/bin/ykksm-gen-keys - from package: yubikey-ksm

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

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