tasks-mobile: 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/tasks-mobile - from package: tasks-mobile

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

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