Wix Stores is two apps, not two versions
The product page in a Wix store belongs to one of two different apps. Not two versions of one app — two apps, each with its own id, and a site runs one or the other.
Each widget id has to be paired with its own app id. Declare the old product page’s widget under the new app’s id and you have named a placement that exists nowhere at all.
Here is the part that cost days: nothing reports it. autoAdd finds no
host app to attach to and silently places nothing. addSitePlugin fails with
a generic message that reads exactly like a Wix outage. The dashboard button
reports failure on a site that is completely healthy.
We carried that mistake for days, with both placements using the new app’s id while the development site ran the old Stores. Every symptom pointed somewhere else — the plugin never appearing looked like a placement bug, and the button reporting failure looked like a platform problem.
A site’s version is visible in its page source: search it for either app id. The fix is to ship placements for both, correctly paired, and let whichever one the site has bind.
The general lesson is one this project keeps relearning. When an API answers “you asked for something that does not exist” and “something went wrong at our end” with the same response, you cannot debug by reading the error. You have to go and find the ground truth yourself.