Issues can not be associated to Epics. But there is an alternative, Yodiz provides you to create an Epic custom field for ease.

Select and Edit Issue > Custom Field

• Click on Custom field Icon will take you to custom fields listing page.
  

  Add New Custom Field > Select Custom Field Type (Epic) > Add Field details > Define Scope (Select Desired Project) > +Add

• Epic Custom Field added Successfully. Here, you can enter Epic id for reference