wmctrl: 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/wmctrl - from package: wmctrl

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

On Debian possible default full paths are:

/usr/bin/wmctrl - from package: wmctrl

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

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