Since the description field can not be required in Online Express, the only option is to not require the field in Raiser's Edge.
  1. In The Raiser's Edge log in as a supervisor.
  2. Select Configuration, Fields, Honor/Memorial.
  3. Unmark the Required box for Description.
  4. Proceed with processing the donation.
NOTE: This can occur with any of the fields in Config > Fields > Honor/Memorial.  Refer to the error on which field is marked as required that needs to be unchecked to process the Online Express donation.