loopctrl: 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:

/sbin/loopctrl - from package: isdn4k-utils
/usr/sbin/loopctrl - from package: isdn4k-utils

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

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