checkisomd5: 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/checkisomd5 - from package: isomd5sum

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

On Debian possible default full paths are:

/usr/bin/checkisomd5 - from package: isomd5sum

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

Error in other languages:
checkisomd5: bevel nie gevind nie
checkisomd5: comandă negăsită
checkisomd5: comando não encontrado
checkisomd5: commande introuvable
checkisomd5: command not found
checkisomd5: käsku ei ole
checkisomd5: không tìm thấy lệnh
checkisomd5: komanda nerasta
checkisomd5: Kommando nicht gefunden.
checkisomd5: kommandot finns inte
checkisomd5: komut yok
checkisomd5: nie znaleziono polecenia
checkisomd5: níor aimsíodh an t-ordú
checkisomd5: no se encontró la orden
checkisomd5: no s'ha trobat l'ordre
checkisomd5: opdracht niet gevonden
checkisomd5: parancs nem található
checkisomd5: perintah tidak ditemukan
checkisomd5: príkaz nenájdený
checkisomd5: příkaz nenalezen
checkisomd5: команда не найдена
checkisomd5: командата не е открита
checkisomd5: 命令找不到
checkisomd5: コマンドが見つかりません