When you create new investment accounts (pools) in the master chart you associate the assets with fund classes.  You then might run an auto-build routine to add the new account to all the appropriate funds.  Because your intention is not to have the fund you use as an offset fund participating in this new investment, you don't think about adding the fund class for that fund to the master chart.

As a result, after the accounts are built for the donor funds, the offset fund is frequently not created.

When asset rebalancing is next run, when you use an offset, it will possibly need to debit or credit this new asset account in the offset fund.  It might be missing.  You get an error message. To fix the issue just add the account manually to the fund using these steps:

These steps assume the GL account has already been added to the Master Chart.  You cannot add a GL account that is not listed in the Master Chart..
  1. Go to the General Ledger > Accounts > Account Tab.
  2. Click New.
  3. Type the Fund Id in the 5th box (the first large one). 
  4. Enter the natural account in the 6th box.
  5. Click into the next box (or hit Tab) and this will cause the Account Type and Description to populate.
  6. Confirm that these are correct or change if necessary.
  7. Confirm or change the fiscal year.
  8. Save the record to finish.
To easily add additional accounts for the same fund for the same year, click the copy button and change the natural account. Similarly you can add additional accounts for different funds.