Automation : Error while uploading Result

Error while executing Test Run: The default assignee does not have assignable permission or unassigned issues are turned off

How can I re-use the Test Run for multiple platform?

How can I select Test Scenarios/Test Cases not associated with any Stories?

How do I assign Tester to a Test Run?

How to add defects I find?

How to bulk update Result ?

How to export Test Run execution results?

My Test Scenarios have changed after I created Test Run and is not reflected in Test run? How can I update?