wmctrl and firefox problem

CentOS release 6.9 (Final)

SikuliX Package Build: 1.0.1 25AUG2013190532
Hello, this is your interactive Sikuli (rules for interactive Python apply)
use the UP/DOWN arrow keys to walk through the input history
help()<enter> will output some basic Python information
shelp()<enter> will output some basic Sikuli information
CMD>>> Settings.OcrTextSearch = True
CMD>>> PHPSikuliVars = {}
CMD>>> keyDown(Key.ENTER)

CMD>>> keyUp()
CMD>>> PHPSikuliVars["var_1"] = App('Firefox')
CMD>>> PHPSikuliVars["var_2"] = PHPSikuliVars["var_1"].focus()
[log] App.focus Firefox(0) #0
[log] switchApp: Firefox
[error] switchApp: java.io.IOException: Cannot run program "wmctrl": error=2, No such file or directory
[error] App.focus failed: Firefox not found
CMD>>> PHPSikuliVars["var_3"] = PHPSikuliVars["var_2"].window(0)
Sikuli ERROR:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'window'
PHP Fatal error: Uncaught SikuliException: Sikuli ERROR:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'window' in /home/mstrategy/libs/sikuli/PHPSikuli.php:1508
Stack trace:
#0 /home/mstrategy/libs/sikuli/PHPSikuli.php(1484): PHPSikuli->_errorToException('Traceback (most...')
#1 /home/mstrategy/libs/sikuli/PHPSikuli.php(1164): PHPSikuli->_getStreamOutput()
#2 /home/mstrategy/libs/sikuli/PHPSikuliBrowser.php(651): PHPSikuli->callFunc('window', Array, 'PHPSikuliVars["...', true)
#3 /home/mstrategy/libs/sikuli/PHPSikuliBrowser.php(595): PHPSikuliBrowser->_getBrowserWindow('firefox')
#4 /home/mstrategy/libs/sikuli/PHPSikuliBrowser.php(137): PHPSikuliBrowser->_setBrowser('firefox')
#5 /home/mstrategy/daemon.php(5): PHPSikuliBrowser->__construct('firefox')
#6 {main}
  thrown in /home/mstrategy/libs/sikuli/PHPSikuli.php on line 1508