check_postgres_disk_space: 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/check_postgres_disk_space - from package: check-postgres

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

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