Using WordPress + Oxygen builder and can not found featured image in post/page settings? It happend to me after WordPress 5.7 update. If you didn’t disabled it in gutenberg settings, then you might have same problem as me.
How to fix it?
Install/re-Install theTwenty Twenty-One theme to your website and your featured images will be back! It is propably because Oxygen does not use the templates. I delete all themes every time I use Oxygen. And WordPress somehow realize this and disable featured image.
If you have Twenty Twenty-One installed already hten delete it via Appearance > Themes and then reinstall the same theme (all without disabling Oxygen). The Featured Image box was then visible again.