This warning message will appear to notify you that the closing balance from the prior financial year (Box 7) does not match the opening balance for the current year (Box 1).
Make sure you have closed the previous year's accounts and copied the annual returns data to the next financial year. In the previous financial year go to Year End/Close Accounts, ensure all boxes are ticked and 'Confirm'.
If your Box 7 still does not match your Box 1, you can check the opening balances for the current year under Setup/Bank Balances. The figures here should be your "cashbook" balances (i.e. bank balance as at 31st March, less unpresented payments, plus unpresented receipts). If these are incorrect, you can copy forward your cashbook balances in the current year via Setup/Bring Forward Balances.