realmctl: 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/realmctl - from package: dnssec-tools

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

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