The reason why you are getting this message in your ''Interview requests'' (Dashboard > Interviews > choose appropriate Admission round > Interview requests) tab is because the deadline for the applicants to submit their interview preferences has not passed yet. 

Applicants are still submitting their invitations and supervisors haven't had the chance to accept them yet, hence why you cannot see any interview requests under that tab and do not have the option to send new interview invitations.

Once the deadline passes, you will see which interviews have been requested/accepted by both a supervisor and applicant, and will have the option to add additional interviews thus pairing up any supervisor with any applicant.  

Related articles: How can I invite additional applicant for interviews? 
Why can't I see which applicants invited me to an interview?