How to update the status field of a standard field (status) in Quotation doctype

I want to change the status of a Quotation, changing the status to “Sales Invoice” when a Sales Invoice is submitted and it seems that the status field has fixed options which are: Draft, Open, Replied, Partially Ordered, Ordered, Lost, Cancelled, Expired but I want to use a custom text “Sales Invoice”. Is there anyway I can do that?

Below is my error

Below is my code