I had this problem with an iPhone app, and fixed it using the following steps.
- With your device connected, and Xcode open, select Window->Devices
- In the left tab of the window that pops up, select your problem device
- In the detail panel on the right, remove the offending app from the “Installed Apps” list.
After I did that, my app rebuilt and launched just fine. Since your app is a watchOS app, I’m not sure that you’ll have the same result, but it’s worth a try.