I want to restrict the assignee in the following manner -

[Joby John]

1. After the fix version is assigned by PO. 

2. Only members from the project role or Current Reporter of the ticket to have the ability to assign the ticket to other developer. 

How can we achieve the same using - Jira Automation or Workflow. Please let me know.

[Mark Segall]

Hi @Joby John - You would probably need to incorporate this into the workflow rather than attributing to when the fix version is applied.  You could establish a new status in your workflow using the jira.permission.assign property on the status.  This would allow you to explicitly set who can reassign the issue.  Here's more info

https://confluence.atlassian.com/servicedeskcloud/useful-workflow-properties-for-jira-cloud-1097176552.html