This Knowledge Base Article: How to make Epic Link required on JIRA Software issue create screen says one should use the 'Fields Required' validator to make Epic Link required. There is no such validator in jira however. At least not in our version 7.11.2.
Is an app required for this validator?
In such cases we usually refer to Add-ons, this one will give you extra validators https://marketplace.atlassian.com/apps/5048/jsu-suite-utilities-for-jira?hosting=datacenter&tab=overview (Jira Suite Utilities)
you will find (Fields Required (JSU)) after adding it to your instance
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.