dh_phpcomposer: 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/dh_phpcomposer - from package: pkg-php-tools

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

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