This happens when trying to adjust a pledge with $0 installment amount (usually seen when entering an SG pledge).

Here’s a workaround you can use:
 
1) Temporarily set the old installment amount to 1 from the backend (ask Team Approach support to do this part)
2) Make and post adjustments
3) Change the installment amount on the old pledge back to 0. (ask Team Approach support to do this part)

Here's an example of the query you can use to track down the previous SG in question, just plug in the account_id or account_ids:
 
SELECT INSTALLMENT_AMOUNT, PLEDGE_STATUS, T.*
FROM   CURRENT_TRANSACTIONS T
WHERE  GIFT_KIND = 'SG'
      AND TRANSACTION_TYPE = 'PL'
      AND ACCOUNT_ID IN (109143, 381348, 1296903, 210879)
ORDER  BY GIFT_DATE DESC;

SG stands for Sustaining Gift