zubair khan

Try Updating PHP

Try adding
define(‘AUTOSAVE_INTERVAL’, 120 );
to wp-config.php file.

The Error arise only When Your ssl is expire or mis-configured.

If you received an error, it means the WordPress Rest API is disabled. You can enable it by activating your permalinks in WordPress. Visit Settings ? Permalinks within WordPress and without making any changes click Save changes. This causes WordPress to flush its rewrite rules, and can often resolve issues like this.

Try to avoid External resources and Add the following to your .htaccess file:
# compress text, html, javascript, css, xml:
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/javascript
AddOutputFilterByType DEFLATE application/x-javascript

# Or, compress certain file types by extension:
<files *.html>
SetOutputFilter DEFLATE
</files>`
otherwise try opcache_enabled

reinstall wp-fancyzoom

Make sure You have connected database Successfully .
Search your wp-config

// ** MySQL settings - You can get this info from your web host ** //
/** The name of the database for WordPress */
define( 'DB_NAME', 'database_name_here' );
/** MySQL database username */
define( 'DB_USER', 'username_here' );
/** MySQL database password */
define( 'DB_PASSWORD', 'password_here' );
/** MySQL hostname */
define( 'DB_HOST', 'localhost' );