ppmtops: 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/ppmtops - from package: xpaint

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

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