marc1964 on "fatal error"

installed the " jet pack plug in " and enabled it and this happened
Fatal error: Call to undefined function is_customize_preview() in /websites/123reg/LinuxPackage22/el/it/eg/elitegamer.co.uk/public_html/wp-content/plugins/slimpack/slimpack/modules/widgets/contact-info.php on line 38 and i have just started a radio advertising campaign today using this web site