A test session can be started from a requirement's test cases page. In the details window of the specific requirement, click on the test cases icon on the right side vertical panel. There will be a Start Test Session button that can be clicked to begin testing the requirement.
In order for the button to be accessible, make sure of the following:
- requirement version is final
- there is at least one linked test case with final status
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.