ppmcombine: 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 Debian possible default full paths are:

/usr/bin/ppmcombine - from package: gerris-mpi
/usr/bin/ppmcombine - from package: gerris

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

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