Before you can delete the Applicant you need to delete any Applicant actions.
To do this, please follow the below steps:
Once on the Job, go to the Applicant Action Tab.
If it doesn't show, click the three dots on the far right of the Tabs and select it.
Click the Eye symbol to view the applicant action record.
Click the More button.
Click Delete.
Click Yes in the dialogue box that appears.
Once the applicant action is removed you should be able to remove the applicant.