Why I can't delete the sales invoice

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Hi All,

Could you help check this issue. I use the sample company 'ABC' to testing sales invoice creation. But when I delete the invoice '0100057,the invoice still appears in the invoice list. And then try to select this deleted invoice to delete again, the 'Delete' button is gray and disable. How can I delete these invoices? If I use another sample company 'ABX', these unnecessary invoice can be deleted correctly. Is there any settings for the deleting operations?

Thanks,

Derek