This Tip is a Troubleshooting on Ubuntu/Debian apt-get install 404 Not Found Package.
The Content give Focus Only to the Essentials Instructions and Commands to make it Easier to Follow.
Solution:
apt-get update
$schemamarkup = get_post_meta(get_the_ID(), 'schemamarkup', true); if(!empty($schemamarkup)) { echo $schemamarkup; }