Hi,
You can only set warmupData in the backend. If you want to get rid of the error, you can wrap the setting with
if (wixWindow.rendering.env == "backend")
as it’s shown in the code example here: https://www.wix.com/velo/reference/wix-window/warmupdata-obj/set
For more information, check out the post about this API here: https://www.wix.com/velo/forum/news-announcements/new-and-improved-warmup-data-api