apt-mirror: 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/sbin/apt-mirror - from package: apt-mirror

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

On Debian possible default full paths are:

/usr/bin/apt-mirror - from package: apt-mirror

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

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