Upsell Popup not working

Hi,
I want to setup an upsell popup, but it’s not working, it doesn’t appear. Am I missing something?

An explanation could be the side cart of the theme that opens when a user clicks on the “Add to cart” button.
https://blumenkultur.quantenfrosch.at/produkt/kranz-fuer-die-tuer-lila-creme/

Any idea?

Thanks in advance,
Robert