When a quotation is valid until certain date, if that date is the current date, system does not allow to create sales order.
In that case, I need to cancel quotation, then modify quotation, and assign a validity to the next day. that way, I can generate sales order.
For example, I have a quotation valid until 20th december 2019. Today is 20th december so the system does not allow me to create the sales order.