Constituents that have only one email address will have that email marked as the Primary email.  When an email is marked as primary, end dates cannot be added.  This is also true for addresses or phone numbers.

To enter an end date, you must add an updated email address and mark it as the primary email.
If you do not have a new email address, you can either delete the old one, or edit the email address and mark the 'Do not email" checkbox