apper: 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/apper - from package: apper

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

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