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

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

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