About 404 page

Hi Mamun

you can remove the sidebar from the 404 with this snippet:

add_filter( 'generate_sidebar_layout','tu_custom_search_sidebar_layout' );
function tu_custom_search_sidebar_layout( $layout )
{
 	// If we are viewing 404 message, set the sidebar
 	if ( is_404() )
 	 	return 'no-sidebar';

 	// Or else, set the regular layout
 	return $layout;

 }

https://docs.generatepress.com/article/adding-php/

Or you can use this CSS:

.error404 .sidebar {
    display: none;
}