This error occurs if you have attempted to install the plugin on more domains than your license allows. The free version of PushButton Optimizer (PushButton Lite) allows you to install the plugin on a single domain. The premium version of the plugin has several different licensing options allowing multiple installations.
If you wish move the plugin to a different domain and have no licenses remaining you can deactivate one of your licenses by clearing the Activation Code field in the General Settings screen and clicking Save Settings. This will free one of your licenses so you can install the plugin elsewhere.
If you have reached your license limit and need further licenses you will need to purchase another copy of the plugin with the number of additional licenses you require.
Please see http://pushbuttonseo.com for purchasing options.