Redirect domain.com/post-name to domain.com/blog/post-name

function custom_redirect_404_to_blog() {
    if (is_404()) {
        // Get the current URI without the home URL
        $requested_uri = $_SERVER['REQUEST_URI'];

        // Check if the URI does not already start with '/blog/' to avoid infinite redirection
        if (substr($requested_uri, 0, 6) !== '/blog/') {
            // Construct the new URI by prepending '/blog' to the requested URI
            $new_uri = '/blog' . $requested_uri;

            // Perform the redirection to the new URI
            wp_redirect(home_url($new_uri), 301);
            exit;
        }
    }
}
add_action('template_redirect', 'custom_redirect_404_to_blog');