I want to change the assignable users according to the issue type. For example;
I have groups such as Product owner (PO), dev, team leader(TL).
Issue type= EPIC, then only one user from the PO group can be the assignee.
Issue type= User Story, then only one user from the PO and TL group can be the assignee.
Issue type= Task, then only one user from the PO, TL and DEV group can be the assignee.
When I select EPIC issue type, system should not let me to select one DEV group as assignee.
Welcome to the Atlassian Community.
You would have to use an app in order to do this, like scriptrunner or power scripts. See these threads for more information, https://community.atlassian.com/t5/Jira-Service-Desk-questions/How-can-I-limit-the-Assignee-field-based-on-the-other-users-of/qaq-p/913814 and https://community.atlassian.com/t5/Marketplace-Apps-Integrations/Can-Scriptrunner-limit-the-Assignee-list-based-on-another-field/qaq-p/712101
I did a quick search but could not find the exact script you are looking for, but if you are using Scriptrunner then have a look at their documentation, https://scriptrunner.adaptavist.com/5.3.0/jira/recipes/behaviours/restricting-priority-and-resolution.html. There is also an Adaptavist collection here on the community.
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