Hello I'm trying to change some configuration in my rss files but I need permission 777 in the html/wp-includes directory. My rss is not showing images but other media files. Do you know if this is doable or I need my own server (not hosted site)
Probably not doable.. You never want to have wp-includes be 777. That's a core folder and would be a security issue.
If something is not working as it should, that could indicate a conflict -- usually a plugin. Make sure everything is up to date and that the plugins you are using (and your theme) are compatible with the latest version of WP (4.9.8).
If everything is up to date, deactivate all your plugins to see if the issues resolves itself. You can then reactivate your plugins one-by-one until you discover the bad actor.