guestmount: 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/guestmount - from package: libguestfs-tools-c

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

On Debian possible default full paths are:

/usr/bin/guestmount - from package: libguestfs-tools

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

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