ppdmerge: 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/ppdmerge - from package: cups

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

On Debian possible default full paths are:

/usr/bin/ppdmerge - from package: cups-ppdc

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

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