Try the following steps in order until the issue is resolved:

  1. Add the new account segment code in Configuration or on the fly.
  2. Verify the account segment code is active:
    1. In Configuration, click Tables.
    2. Highlight the segment table and open the table entry.
    3. Unmark the Inactive checkbox and click OK.
  3. Verify the account segment combination is valid:
    1. In Configuration, click Account Setup.
    2. Highlight Invalid segment combinations.
    3. Remove the invalid segment combination.
    4. If the segment combination should be invalid, select a different combination when creating the account.