The Chart of Accounts needs to have an Retained Earnings account (usually an equity account) that has the retained earnings check box checked on the account card.

If you have a retained earnings account, open up the account by navigating to Accounting --> Chart of Accounts and double clicking the account to open it. Next, check the check box labeled Retained Earnings. Click Save and Close and re-run the Trial Balance Report.

If you do not have a retained earnings account, create one by navigating to Accounting --> Chart of Accounts and clicking create. Use Equity as the account type, name the account and check the Retained Earnings check box. Click Save and Close and re-run the Trial Balance Report.