I asked with chatgpt and i got this reply ..
Yes, this is a very common Osclass issue 👍
The “Forbidden” error when saving Google AdSense code in Themes → Advertisement is not an Osclass bug. It is almost always caused by server security (ModSecurity / WAF) blocking AdSense <script> code.
I am using Hostinger. and also chatgpt suggest ad codes in item.php so i have pasted but when i will update theme, I have to change this setting again...