This is because you have activated the Google Captcha plugin but not set it up. If you don't want to use a captcha on the login form, go to Plugins > Installed Plugins and deactivated the Google Captcha plugin. 

If you do want to use this plugin, you'll need to set it up. Go to Google Captcha > Settings and click the Get the API Keys link to generate a site and secret key which you need to add to the plugin.