aegisub-3.1: 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/aegisub-3.1 - from package: aegisub

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

Error in other languages:
aegisub-3.1: bevel nie gevind nie
aegisub-3.1: comandă negăsită
aegisub-3.1: comando não encontrado
aegisub-3.1: commande introuvable
aegisub-3.1: command not found
aegisub-3.1: käsku ei ole
aegisub-3.1: không tìm thấy lệnh
aegisub-3.1: komanda nerasta
aegisub-3.1: Kommando nicht gefunden.
aegisub-3.1: kommandot finns inte
aegisub-3.1: komut yok
aegisub-3.1: nie znaleziono polecenia
aegisub-3.1: níor aimsíodh an t-ordú
aegisub-3.1: no se encontró la orden
aegisub-3.1: no s'ha trobat l'ordre
aegisub-3.1: opdracht niet gevonden
aegisub-3.1: parancs nem található
aegisub-3.1: perintah tidak ditemukan
aegisub-3.1: príkaz nenájdený
aegisub-3.1: příkaz nenalezen
aegisub-3.1: команда не найдена
aegisub-3.1: командата не е открита
aegisub-3.1: 命令找不到
aegisub-3.1: コマンドが見つかりません