a2dissite: 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/a2dissite - from package: apache2

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

Error in other languages:
a2dissite: bevel nie gevind nie
a2dissite: comandă negăsită
a2dissite: comando não encontrado
a2dissite: commande introuvable
a2dissite: command not found
a2dissite: käsku ei ole
a2dissite: không tìm thấy lệnh
a2dissite: komanda nerasta
a2dissite: Kommando nicht gefunden.
a2dissite: kommandot finns inte
a2dissite: komut yok
a2dissite: nie znaleziono polecenia
a2dissite: níor aimsíodh an t-ordú
a2dissite: no se encontró la orden
a2dissite: no s'ha trobat l'ordre
a2dissite: opdracht niet gevonden
a2dissite: parancs nem található
a2dissite: perintah tidak ditemukan
a2dissite: príkaz nenájdený
a2dissite: příkaz nenalezen
a2dissite: команда не найдена
a2dissite: командата не е открита
a2dissite: 命令找不到
a2dissite: コマンドが見つかりません