installsieve: 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/installsieve - from package: cyrus-imapd-utils

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

On Debian possible default full paths are:

/usr/bin/installsieve - from package: cyrus-admin

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

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