@RS81, you can in fact delete the invoice if really necessary. But to do so, you must first “unwind” the entire string of transactions. So you must delete the receipt of payment first. That is the “object” to which the warning message refers. Once all such dependencies are eliminated, you will be able to delete the sales invoice.
However, it is a poor idea to delete an invoice you have already sent to a customer. If they overpaid you as a result, issue a credit note. If they underpaid you, issue another sales invoice for the additional amount and explain what happened to the customer.
The program is designed to preserve data and minimize the possibility of accidental deletion. Imagine how upset you would be if you clicked a Delete button instead of the nearby Update button and lost your record. But anything in Manager can be edited and updated; default fields can be overwritten; etc.