ppmtosixel: 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/ppmtosixel - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtosixel - from package: netpbm

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

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