In the requirement's right side vertical menu, click test cases icon and then click the New/Link button. Select + New Test Case from the dropdown, enter a test case title and click Save. Click on the test case ID to open the details and edit.
Users will need to have project permissions to view and edit in both Requirements and Test Coverage to perform this action. Depending on the settings of the user's project permissions, they may be able to view all the project requirements and all test coverage items, or just a subset of each.