How to add custom post types to normal category pages

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You have one or two problems here

  • is_category() should be object of $query in your example code

  • get_query_var('post_type') will always return false on a category page AFAIK, so that code is totally unnecessary

  • Just a tip, when using pre_get_posts with any type of archive, also check for non admin pages as your back end will also be affected by this change

You can try something like this

add_action( 'pre_get_posts', function ( $q ) {
    if( !is_admin() && $q->is_main_query() && $q->is_category() ) {
        $q->set( 'post_type', array( 'post','event' ) );
    }
});