Hello,

Please check how it works with the default WordPress theme just to check whether the issue is with our theme or not.

Because the upsells are default WooCommerce functionality and our theme doesn’t influence.

If it works fine with the default WordPress theme then deactivate all the third-party plugins and provide with the admin panel login details of the website then we will troubleshoot with the theme plugins.

Regards.
Xtemos Studios.