Yes, the field you are attempting to update is a sub field, which I’ve gone over already.
You can’t use update_field() to update a sub field of a group field. At a minimum you should be using update_sub_field(). This function requires passing an array of the field hierarchy as the selector/field_name
update_sub_field(array('field_64b38c4e3ec56', 'field_64c782fc4c55d'), $value, $post_id);