aria2c: 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/aria2c - from package: aria2

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

Error in other languages:
aria2c: bevel nie gevind nie
aria2c: comandă negăsită
aria2c: comando não encontrado
aria2c: commande introuvable
aria2c: command not found
aria2c: käsku ei ole
aria2c: không tìm thấy lệnh
aria2c: komanda nerasta
aria2c: Kommando nicht gefunden.
aria2c: kommandot finns inte
aria2c: komut yok
aria2c: nie znaleziono polecenia
aria2c: níor aimsíodh an t-ordú
aria2c: no se encontró la orden
aria2c: no s'ha trobat l'ordre
aria2c: opdracht niet gevonden
aria2c: parancs nem található
aria2c: perintah tidak ditemukan
aria2c: príkaz nenájdený
aria2c: příkaz nenalezen
aria2c: команда не найдена
aria2c: командата не е открита
aria2c: 命令找不到
aria2c: コマンドが見つかりません