Try the following steps in order to resolve the issue:
  1. Go to Marketing and Communications > Appeal Mailings.
  2. Click on the specific mailing.
  3. Go to the Letters tab, select a letter, and click Edit.
  4. In the section "Selections of constituents that will receive this letter," ensure there is a selection defined here.  If not, go to analysis > information library.  Add a constituents ad-hoc query, mark the box to create a selection, and save the query.  Then select this selection in the letter of the appeal mailing.
  5. If step 4 doesn't resolve, in the "how do you want to send this letter section," see if email is marked. 
  6. Verify you have email content defined.  If it is not defined, click Add and add email content.  You cannot run an email appeal mailing if there is no content, since you do not want to send an email with no content.