You can not do this. Epic Link is a system field (added by Jira Agile/Jira Software). All Epic functionnalities on boards depends on this field so it can not be made required like this.
Locked fields are locked by the plugins or core code that needs to protect them from potentially inappropriate change or deletion.
Unless Atlassian documentation explicitly says "you need to unlock this field", you should not unlock them. Some of them probably could be deleted safely, but most will break things if you were to unlock them and change them, let alone delete!
Could we ask why you think you want unlock the fields? There are a couple of reasons, but I doubt you've actually got one that won't harm your system.
Do not do this. You will break Jira in some "interesting" ways (fortunately non-destructive, you'll just find a whole load of stuff starts failing for no apparent reason)
Use a validator to make the field mandatory on creation, or, if you have Scriptrunner, a Behaviour.
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