What iOS is your phone running?
I had the same emailed message from Discover. I assumed that I had mistakenly removed that card during some maintenance or it was just one of those hiccups (that Apple products don't really have, right?) So I went ahead and added the card to my wallet (again) and ran some errands. When I got home, I had another email from Discover saying the same: that I had removed my Discover card and could reinstall it at any time. I tried immediately again and got the same result.
Now I'm curious, because I'm sure the card/account is still valid, and all other credit cards, debit card and reward cards in my wallet still work. So I called Discover and went through a few levels of customer service and no one could figure it out. One CS person revealed that this was not the first call she had received on the topic. The best that their third level tech person could surmise was that it was due to my not having updated the iOS recently, despite the fact that the card had been working through an older iOS just fine up until that point. It sounded like a shot in the dark response, not really one based on any specific knowledge or tech inquiry. I think she said, "Have you downloaded the latest software update to your phone yet? You might want to try that and see if that helps."
I haven't gotten around to updating yet; I'm also not so inconvenienced by having to pull a credit card out (of the
case that the iphone is already in) so it's kind of a shrug for me personally, although I could see how some folks here would get irritated.