The initializeExtensionTask is only available if the 'PluginInit*.java' was not created. So make sure the file does not exist or download and create a new empty project.
It already solved. I try to re unzip the file (rapidminer template master) and run it (initializeExtensionProject) from eclipse, it gave me an error. Then I try to re unzip again and run it from command and it run well, after that I try to re unzip again and run it from eclipse, it run well too. I don't now what's wrong, but it solve my problem.