The theme you’re trying to use as a parent, Grow Minimal, is a child theme of the Grow theme. It’s not possible to create a child theme of another child theme without writing custom code workarounds.
In your code, these two lines just do the same thing twice:
wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
wp_enqueue_style('thinkup-style', get_template_directory_uri() . '/style.css');
They enqueue the stylesheet for the parent theme, Grow. The Grow Minimal theme is ignored. This question provides more context about the problems with grandchild themes.
In this case, the best practice is to simply add your custom styles or template code directly to the off-the-shelf child theme (Grow Minimal), effectively converting it to a custom theme. You’ll still be able to apply any updates to the unmodified parent theme without losing your customizations.