Some budget shared hosts disable SSI globally for security or performance reasons. Open a support ticket and ask: "Do you support Server Side Includes (SSI) for .shtml files? If so, can you enable the 'Includes' option for my directory?"
If you are on a shared Linux hosting environment (like cPanel), you likely do not have access to the main server configuration files. You can fix this by adding directives to your local .htaccess file found in your site's root directory ( public_html ). view shtml fix
After applying changes, restart Apache: