obm-nav: command not found

Have You tried accessing the binary with the full path?

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/obm-nav - from package: obmenu

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