Horizontal Menu in Constructor Theme

hi,

even after create a ‘Header’ menu in Appareance Menu and added it to Top Sidebar in Appareance, Widget Menu, when i activate Horizontal Menu in Constructor Option Header Options i get:

Fatal error: Call to undefined function has_nav_menu() in /home/wexpress/public_html/wp-content/themes/constructor/libs/Constructor/Main.php on line 193

can anybody explain me what’s missing??