mailman-api: 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/mailman-api - from package: mailman-api

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

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