The 3900 Retained Earnings account does not show up on the Trial Balance report in Core. This is by design because a trial balance shows only accounts that have actual transactions in the accounting tables. In Core, we don’t create any entries for this account as it is handled automatically upon closing of books.
To show this account in your Trial Balance report, you need to post entries to that account. At the year-end, you must close your nominal accounts (revenue, income and expense) and transfer their balances to the Retained Earnings account. Please read this article for more on that.