This has been an issue for a couple years now, but TCINs, for some reason, can't be searched in myDay. This makes making/reprinting barcodes and defect stickers for Target+ stuff a pain at Guest Service, as our HIP Printers love to just not work for no reason. A workaround that we've found at my store is to search the TCIN on Target.com/Target app, find the UPC for the item via the item page, and copy-paste/transcribe the UPC from Target.com to myDay, which is very tedious.
IDK when this happened tbh. Because I feel like a few years ago, you could input a TCIN into the myDay search and it would pull up the item you're looking for, wether regular merchandise or a Target+ item.