This problem can be caused by a privacy setting on iPhones.
Go to Settings > Screen Time > Content & Privacy Restrictions
If Privacy Restrictions are turned on, make sure that "Contacts" is set to allow.
If "Contacts" is already set to allow, try turning off Content & Privacy Restrictions and see if the problems still occur.
If the problem still occurs, delete TappTracker and reinstall. by going to Settings > General > iPhone Storage > TappTracker > Delete App.