You can create a new defect or link an existing one to a test execution or to a test execution step.
At the test-execution level, use one of the following two options in the Issues section.
At the test execution step-level, you can create a new defect or link an existing one to any test step by clicking the cog icon, then selecting Create Issue or Search Issues.
If you click Create Issue, fill out your desired fields on the Create Issue screen, then click Create to link the new issue to the test step.
You can see the linked defect in both the test-case list and the step.
Links also appear on the defect in the Impacted Test Executions section.