dropdb: 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/dropdb - from package: postgresql

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

On Debian possible default full paths are:

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

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

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