If you'll be setting up Guardian Consultations in bulk for guardians to book onto, any students you wish to run Parents Evenings for need to be currently enrolled students.
When students have Applicant status they aren't enrolled - they only show up in the Applicants section of your site. You can see how to enrol them here.
However, if the Guardian Consultation date is set to a date after their enrolment date, there is a way to run Guardian Consultations with them. You need to use the Custom Meeting option.