Watch app on iPhone won’t open??

Try out my suggestion which doesn’t require a factory reset like others are suggesting. It is doable, you just have to be quick. This has worked for many other people. See instructions below:

1. On the watch go to settings > general > reset > Erase All Content and Settings
2. Go to your Bluetooth settings on your phone and if you see your Apple Watch, click on the 'i' and click "Forget this device"
3. Go to "appleid.apple.com", scroll down to devices and you should see your apple watch. Click your apple watch and select "Remove from account".
4. Next you will need to turn off "Find my iPhone", which should turn off activation lock on the watch. On your phone go to settings > click your name > Find My > Find My iPhone > toggle this off
5. Next step is to go go settings > general > iPhone storage > scroll to the Watch app and "Remove all backups". I saw someone do this somewhere else, saying that there might be corrupt files in the backups, so removing them might be a good idea. If you do have a backup and want to try without this step, it might still work, just wanted to mention this.
6. These next steps have to be done quickly. Do a Hard reset of your phone (quickly press volume up > quickly press volume down > press and hold side button until Apple logo appears and then release side button). Once the phone turns back on, you should have enough time to be able to get into the app and then unpair the watch, but you have to do this super quickly before the app kicks you out. Make sure your app is on the first screen once you unlock your phone, because you don't have much time from when the phone turns on from the hard reset to when the app will crash. I also recommend disabling your passcode if you have one to get into your phone quicker. 
7. Next, go into the app, and once loaded, quickly press "All Watches" in top left screen > Click the 'i' next to your watch > then click "Unpair Apple Watch".
8. After you click "Unpair Apple Watch", a pop up at the bottom appears where you have to click “Unpair Apple Watch” again. After you have done this, you should see a screen where it shows it is unpairing your watch. Because you turned off Find my iPhone in the previous step, this should have turned off the activation lock, so you shouldn't have to type in your Apple ID password. Once I got to this point, the app didn't kick me out and I was successfully able to unpair my watch.
9. After the unpair is completed, go back to your apps and you should have the ability to completely delete the Watch app. First remove the app from your home screen and then completely delete the app from the app library.
10. After you have deleted the app, go back to the app store and download the watch app again and you will be able to get into the app with no issues and start over with your watch.

Since I did this, my watch has been working perfectly. Let me know if this works for you!