slon_start: 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/slon_start - from package: slony1-2-bin

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

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