ppm2tiff: 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/ppm2tiff - from package: libtiff

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

On Debian possible default full paths are:

/usr/bin/ppm2tiff - from package: libtiff-tools

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

Error in other languages:
ppm2tiff: bevel nie gevind nie
ppm2tiff: comandă negăsită
ppm2tiff: comando não encontrado
ppm2tiff: commande introuvable
ppm2tiff: command not found
ppm2tiff: käsku ei ole
ppm2tiff: không tìm thấy lệnh
ppm2tiff: komanda nerasta
ppm2tiff: Kommando nicht gefunden.
ppm2tiff: kommandot finns inte
ppm2tiff: komut yok
ppm2tiff: nie znaleziono polecenia
ppm2tiff: níor aimsíodh an t-ordú
ppm2tiff: no se encontró la orden
ppm2tiff: no s'ha trobat l'ordre
ppm2tiff: opdracht niet gevonden
ppm2tiff: parancs nem található
ppm2tiff: perintah tidak ditemukan
ppm2tiff: príkaz nenájdený
ppm2tiff: příkaz nenalezen
ppm2tiff: команда не найдена
ppm2tiff: командата не е открита
ppm2tiff: 命令找不到
ppm2tiff: コマンドが見つかりません