fail2ban-client: 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/bin/fail2ban-client - from package: fail2ban

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

On Debian possible default full paths are:

/usr/bin/fail2ban-client - from package: fail2ban

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

Error in other languages:
fail2ban-client: bevel nie gevind nie
fail2ban-client: comandă negăsită
fail2ban-client: comando não encontrado
fail2ban-client: commande introuvable
fail2ban-client: command not found
fail2ban-client: käsku ei ole
fail2ban-client: không tìm thấy lệnh
fail2ban-client: komanda nerasta
fail2ban-client: Kommando nicht gefunden.
fail2ban-client: kommandot finns inte
fail2ban-client: komut yok
fail2ban-client: nie znaleziono polecenia
fail2ban-client: níor aimsíodh an t-ordú
fail2ban-client: no se encontró la orden
fail2ban-client: no s'ha trobat l'ordre
fail2ban-client: opdracht niet gevonden
fail2ban-client: parancs nem található
fail2ban-client: perintah tidak ditemukan
fail2ban-client: príkaz nenájdený
fail2ban-client: příkaz nenalezen
fail2ban-client: команда не найдена
fail2ban-client: командата не е открита
fail2ban-client: 命令找不到
fail2ban-client: コマンドが見つかりません