tasksh: 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/tasksh - from package: task

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

On Debian possible default full paths are:

/usr/bin/tasksh - from package: taskwarrior

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

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