Hello.
How exactly are you trying to create new site, please? Via "New site wizard" in Site Manager -> Sites (so the installation of Kentico CMS´ instance was successful), or during installation procedure of new Kentico
CMS installation? Or are you trying to import some existing site via import procedure?
If you have functional instance, could you please check your Site Manager -> Development -> Document types section, if document type of cms.menuitem class name exists here? Thank you.
Best Regards,
Radek Macalik