Create Page with Template File from Plugin Folder

It is probably wp_insert_post can’t recognize your template. You need to include it in the list of the templates.

function wpse410645_add_plugin_template( $page_templates ) {

    $page_templates[plugin_dir_path( __FILE__ ) . '/templates/mytemplate.php] = 'My Template Name';
    
    return $page_templates;

}
add_filter( 'theme_page_templates', 'wpse410645_add_plugin_template' );