Can't add item to watch list from Android phone, says "something went wrong, item not added to watch

[*sons_and_daughters*]

As a test, I just added an item. No issues. Have you tried updating the app, or uninstalling and reinstalling? I find that usually fixes most problems. You could also go into your app settings and clear the cache and cookies. Reinstalling is usually enough though. For me anyway.

[eburtonlab]

Make sure you have not reached the maximum number of items (400?) -- that includes all active as well as ended items, so be sure you are looking at the total list rather than just a subset. Try deleting some old items and see if that allows you to add new ones.

Not sure about the eBay app, but when using a browser, some Watch List issues can be helped by signing out of eBay, restarting the browser and signing back in.

If the problem is with one particular item, and other items can still be added, perhaps that one item has an issue -- such as being in the process of being removed by eBay.