wordpress how to activate 404 page if a post if not found code example

Example: wordpress inner page not found error

NOTE : If you got this error after installing wordpress on a newly configured 
ubuntu server
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1. sudo a2enmod rewrite

2. Add AllowOverride All in virtual host
<Directory /var/www/html>
    Options Indexes FollowSymLinks
    AllowOverride All
    Require all granted
</Directory>

3. restart apache