Follow the the below options in order:
Option One: Unpost and Repost Entries
Ensure a Backup has been taken of the Ledger.
Go to Data Entry.
Select Unpost All Entries.
Select OK to both pop up messages.
Go to Data Entry.
Select Post Entries.
Ensure the Period Range is correct. By default the To criteria will need to be adjusted.
Click Post, then Yes.
Close the window.
Go back to the Trial Balance and see if the error has cleared.
Option Two: Check Accounts
Check that there are Main Accounts existing for all Sub Accounts.
Once added, go back to the Trial Balance and see if the error has cleared.
Option Three: Send to Support
If the previous steps have not worked please raise a new case online, and reference the title of this article.
Please attach a copy of the ledger to the case.