By default [wc_1cu_yes] and [wc_1cu_no] shortcodes display only a special link in a text form, not as an active link.
So there are 2 ways of using the shortcodes on your custom upsell offer page:
1. You need to enter [wc_1cu_yes] into a link element: <a href="[wc_1cu_yes]" ...>Your YES link</a>
Important: please, doublecheck that there is NO "http://" prefix before the shortcode (ex. "http://[wc_1cu_yes]")
2. Alternatively you can wrap an image or a text into the shortcode to get it working, then the shortcode will be transformed into a link automatically and you won't need to insert it manually:
[wc_1cu_yes] Your image button or just "Yes text" [/wc_1cu_yes]