Improper Message: A subtask cannot be assigned to an epic

Summary

When creating a Sub-task inside an Epic or inside any issue that is linked to an Epic, the message "A subtask cannot be assigned to an epic." is shown, when, in fact, you CAN assign a subtask to an epic. It causes confusion for the users.

Steps to Reproduce

1. Go to the Issue View of a ticket which is assigned to an Epic or to an Epic itself.
2. Open More > Create Subtask.
3. In the "Epic Link" the message is displayed.

Expected Results

It would be better to clarify deeper, or even to hide that message in sub-task creation.