Writer, insert- tables and indexes

Hello!
When I start working with writer. insert- tables and indexes, if I want to change or add (bibliography entry) or delete sth, it is not possible, says it is READ ONLY.

I tried to remove read only by edit- file, and other ways, BUT IT DIOD NOT GO!!!
What is the point, please?
many thanks!

LibreOffice Writer Guide

Chapter 12 · Creating Tables of Contents, Indexes, and Bibliographies

… or explain your issue step-by-step …