Same here on latest (nightly) Odoo Community Edition, as of July 2018. No symbol field. Bug!

Error "The following fields are invalid: Symbol" Cause Symbol field is mandatory so you have to enter symbol for that currency accordingly. then check it and let me know !

The form to create a new currency does not have symbol. The fields on the form am seeing are as follows:

1. Currency:

2. Currency Unit

3. Currency Rate (which is disabled)

4. Currency Subunit

Thats all, there is no "Symbol"

enable developer mode

Same problem for Odoo 12, no symbol field