The reason why it is not editable, is because
Your field is a text box. A reference cannot be changed from a text box. Check whether you can change the widget to a reference selector.
If then you still cannot select anything because it is grayed out or so, I would suggest to have a look into the entity access.
Are you sure the object w/ the attributes you’re trying to edit has been created? The attributes for an empty context object behave like read-only attributes.
Kindly make sure the object of the field which you are trying to edit is created and the access for the fields is in write mode