The problem is that the plugin uses a function available in WordPress 4.4 and you probably have a lower version.
Now to get you out of trouble, you could use FTP to reach your site’s files, navigate to wp-content/plugins/ and rename the directory of the troublesome plugin. This would force it to be deactivated. Now you could update your WordPress version to the latest and greatest (I highly recommend this), and then your plugin should work without a hitch.