remove_members: 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 CentOS possible default full paths are:

/usr/lib/mailman/bin/remove_members - from package: mailman

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/lib/mailman/bin/remove_members - from package: mailman
/usr/sbin/remove_members - from package: mailman

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

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