This error is displayed because you have changed the permalinks recently or the portfolio slug.
Go to permalinks and set to ‘Default’, save. Then set again to ‘postname’ or the permalink you want and save again. This will do the fix.
This will force WP to rewrite the .htaccess file and to save the new rewrite rules in the database.
If this doesn’t work, it’s because the .htaccess file isn’t writable.
These are the mod_rewrite rules you should have in your .htaccess file:
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
This is an article about how to test if mod-rewrite is on.
https://www.tutorio.com/tutorial/enable-mod-rewrite-on-apache
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