If you haven't used the Promise section of FIMS before, it is possible that it was never initialized properly.
Promises have numbers just like gifts and grants.  To set the next number:
  1. Tools > System Utilities > Set Automatic Numbers
  2. Choose Promise ID Code
  3. On the right, enter a number 1 larger that the value in the Last Promise ID Code used field.
  4. If that field is blank, enter a 1.
  5. Click Save and then Done.
  6. You should be able to add a promise now.