Add a row to your block template

template.html

/* Adds the row block in an HTML template */
<!-- wp:group {"layout":{"type":"flex","allowOrientation":false}} -->
<div class="wp-block-group"></div>
<!-- /wp:group -->

templates.php

// Creates a post template and adds the "row" variation of a core group block
function myplugin_register_template() {
	$post_type_object           = get_post_type_object( 'post' );
	$post_type_object->template = array(
		array(
			'core/group',
			array(
				'layout' => array(
					'type'             => 'flex',
					'allowOrientation' => false,
				),
			),
			array(),
		),
	);
}
add_action( 'init', 'myplugin_register_template' );