sshguard: 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/sbin/sshguard - from package: sshguard

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

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