mtbl_merge: 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/mtbl_merge - from package: mtbl-bin

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

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