Make sure your theme is adding classes to your widgets correctly.
In the register_sidebar() function your before_widget value should have “%2$s” inside the classes. Currently your theme doesn’t support Dynamic widget classes.
example:
[ Moderator Note: Please post code or markup snippets between backticks or use the code button. ]
register_sidebar(array(
'name' => __( 'Sidebar','wpex'),
'id' => 'sidebar',
'description' => __( 'Widgets in this area are used on the main sidebar.','wpex' ),
'before_widget' => '<div class="%2$s">',
'after_widget' => '</div>',
'before_title' => '<h4 class="heading"><span>',
'after_title' => '</span></h4>',
));
have a look at the codex:
https://codex.www.ads-software.com/Function_Reference/register_sidebar