Encoding problems with Greek words

Question

I wrote a whole text in lyx with a latin-9 encoding, but I need the encoding to be utf-8. I change it in the settings but in my essay I used lots of Greek words. The problem is that changing the encoding the result is an error message and if I generate the output, there are not the Greek words I wrote.

Thank you so much

\documentclass[11pt,oneside,english,italian]{book}
\renewcommand{\rmdefault}{cmr} 
\usepackage[latin9]{inputenc} 
\setcounter{secnumdepth}{5} 
\setcounter{tocdepth}{5} 
\usepackage{color} 
\usepackage{array} 
\usepackage{longtable} 
\usepackage{rotating} 
\usepackage{float} 
\usepackage{rotfloat} 
\usepackage{booktabs} 
\usepackage{textcomp} 
\usepackage{multirow} 
\usepackage{forest} 
\usepackage{amsmath} 
\usepackage{makeidx} 
\makeindex 
\usepackage{graphicx} 
\usepackage{setspace} 
\usepackage[authoryear]{natbib} 
\PassOptionsToPackage{normalem}{ulem} 

I post the screenshot of the error message as well