Reserves help for charity accounts. Why can't we use the retained earnings as a sub account?

You're unable to make Retained Earnings as a subaccount since it's a default one, MaLeon18. I'll provide clarification about updating these detail types on the balance sheet. 

A retained earnings account is a system account because it takes part in a company's equity and financial health. Additionally, deactivating or replacing it with another account is unavailable.

If you meant to update the reserves on the balance sheet by making it a parent account and adding the RE as the subaccount, this option is currently unattainable. As a workaround, you can consider creating another subaccount with distinct characters for easy identification. Alternatively, export the account reports to Excel and summarize them from there. 

Moreover, I recommend consulting your accountant's help for further guidance on managing Retained Earnings and Unrestricted funds to ensure long-term financial stability for the charity.

Furthermore, here's an article to familiarize which accounts you can delete, edit, or merge: Manage default and special accounts in your QuickBooks Online chart of accounts.

Lastly, scan this guide to view your firm's historical profits: View Retained Earnings account details in QuickBooks Online.

Kindly lay your additional questions below if you need further information. It would be my pleasure to assist you. Have a great day.