I started getting this error in the Postie plugin whenever I tried to update it.
Fatal error : require_once(): Failed opening required ‘wp-includes/class-wp-oembed.php’ (include_path=‘.:/usr/share/php’) in wp-content/plugins/postie/postie.class.php on line 24
The latest version requires Wordpress 5.3 meaning it is no longer compatbile with ClassicPress.
Does it? According to the Postie overview on the WP Plugin directory it requires WP4.0 or newer. I have been looking at the forums of the plugin just now, it seems that they tried to make it compatible with WP5.3, but accidentally broke backwards compatibility. I think it was an accident.
Maybe you can revert to the previous version for now, and keep an eye out to see if they fix it.
I think they are going to fix it shortly. class-wp-oembed.php seems to be the renamed copy of class-oembed.php (with just a better indentation ).
So as a temporary fix you could change it in postie.class.php line 24.