mksquashfs: 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/mksquashfs - from package: squashfs-tools

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

On Debian possible default full paths are:

/usr/bin/mksquashfs - from package: squashfs-tools

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

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