From the list view, why can't you sort on a Lookup field? Are there any other suggestions to sort by this Lookup field, Applicant?
Lookup fields are sortable on my end. Can you provide more information about your lookup field like relationship cardinality and screenshots?
Below is a screenshot of my list view wherein the Subject(lookup) is sorted.
If the Applicant lookup is the only field which cannot be sorted in the view, then most probably it is caused by its relationship cardinality.
In my screenshot below, I displayed all the possible relationship cardinality for an object. Only 2 are sortable out of 4: one subject - one student and one subject - many students.
Lookup fields are sortable for 1:1 and 1:N relationships.