Give a try to this-

 var currentSiteDef = SiteDefinition.Current;

Hi,

Scheduled jobs don't really execute in the context of a specific site so I suspect SiteDefinition.Current might not work. I tend to use ISiteDefinitionRepository to get all site instances and just pick one (either a specific one or, if the specific site's not important, the first) like this:

var currentSite = SiteDefinition.Current;
if (currentSite == null || ContentReference.IsNullOrEmpty(currentSite.StartPage))
{
    currentSite = ServiceLocator.Current.GetInstance<ISiteDefinitionRepository>().List().FirstOrDefault();
}
if (currentSite == null)
{
    return "No sites defined";
}

Actually, looking at the comment on SiteDefinition.Current, you can ensure a site definition is always returned by setting a wildcard host mapping (*) in the site configuration in admin mode though it's still worthwhile handling the situation where a site definition can't be resolved.