Customize Proforma Invoice

I just duplicated the DocType Quotation into a Proforma Invoice.
Then I want to edit “Series” to match the doctype name. But why does it always fail and an error like this appears?

Can anyone help explain and provide a solution?

Hi,

Go to Proforma Invoice Doctype and delete one amended_from field out of two fields.

Thanks,

Divyesh Mangroliya