Why can I not edit a sent quote or invoice?

The short answer

Because sent commercial documents are treated as formal history, not just draft notes.

Quotes

Sent quotes do not return to draft. If the client needs changes, the correct path is to create a new quote version and send that revised version.

Invoices

Invoices are intended to be edited while they are still drafts. Once invoicing is active, the platform becomes more protective because the document now represents billing history and payment expectations.

What to do instead

For quotes: create a new version.
For invoices: create or issue the next billing action rather than trying to rewrite a sent record in place.

If you work around document immutability incorrectly, approval and payment history become much harder to trust later.

Revise a sent quote
Create an invoice from an accepted quote
Quotes and invoices