I created a vocabulary in Structure>Taxonomy>Add Vocabulary. Then I add terms to it.
My Vocabulary fields look like this :
I want to display this vocabulary into accounts fields, I want to add existing fields but I can't find the term reference.
You have to use "Add new field" until you haven't used your term reference field for the Departements at least once before.
If have created this field once, you can reuse it for other content types.
You must be wondering why the fields are not appearing in the existing fields, although it has the same table field_data_field_ and has some data in it. And creating another field will create another table which you think would be an overkill.
But if you could look closely at those field data tables which are linked to taxonomies and with nodes you would clearly see the difference in entity types.
So, Drupal can list those field in existing dropdown which has the same entity type as the one you are adding the field to. So in your case they are different. that is why it is not been listed there.
You don't need to Add an existing field, you need to Add a new field
See the picture
Edit (thanks to NoSssweat comment): After you add this field, it will allow you to select your vocabulary.