How can I create A statement of account from the customized reports containing the opening balance
For example, a statement is requested for each asset or a statement for each party, investment or expense account during a certain period, but the allocated reports do not show the opening balance
You cannot. Custom reports draw upon transactions posted to the general ledger. Balances are not posted. However, the Fixed Asset Summary includes that information for fixed assets. Expense account drill-downs can be copied to spreadsheets; they include the necessary information. You need to be more specific about what you mean by “investment.”