dropuser: 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/dropuser - from package: postgresql

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

On Debian possible default full paths are:

/usr/bin/dropuser - from package: postgresql-client-common
/usr/lib/postgresql/9.4/bin/dropuser - from package: postgresql-client-9.4

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

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