post tag texonomy rename getting error

You need to be careful overriding the entire array as WordPress may add new items from time to time…override each specific value instead like this:

add_action( 'init', function() {

    global $wp_taxonomies;

    $wp_taxonomies['post_tag']->labels->name="Groups";
    $wp_taxonomies['post_tag']->labels->menu_name="Groups";
    $wp_taxonomies['post_tag']->labels->singular_name="Group";
    $wp_taxonomies['post_tag']->labels->search_items="Search Groups";

    $wp_taxonomies['post_tag']->label="Groups";

}, 1 );

I showed a few examples just do it for all the values you wish to change.