ykksm-checksum: 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-checksum - 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-checksum - from package: yubikey-ksm

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

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