check_postgres_custom_query: 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_custom_query - from package: check-postgres

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

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