realmchk: 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/realmchk - from package: dnssec-tools

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

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