I had the same issue with a Schlage sensor. Deleted it from the app, in an effort to get it ‘unstuck’, and now I cannot get it added back in. I talked to customer support via the in-app chat, and they mentioned they need to send an “exclude” command while I simultaneously remove the battery from the Schlage and ‘reboot’ it. We tried a couple times, unsuccessfully. Now I cannot even get the app to recognize the sensor anymore.
So moral of the story: do NOT remove/delete the sensor from the app, you will not be able to add it back.