How to make make the following attributes mandatory in Zephyr scale - Objective,Step,Expected result

There's no way to force entry of this requirement, but as an example...

You could create reports and dashboards to help manage and prompt testers.  You could have a 'management' dashboard showing the various  execution status counts for each tester, so you would know which testers have tests Not Executed.  And you could create reports for each tester, that lists there outstanding tests (filter on Results IN Not Executed) - that report is dynamic and will be specific to the tester so each time they run it they can see which tests they haven't executed.