task: 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 CentOS possible default full paths are:

/usr/bin/task - from package: task

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/task - from package: taskwarrior

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

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