Please follow the Troubleshooting steps as given below:

1.   Clear the data of the Google Play Services app. 

 Settings->Apps->Google Play Services->Storage->Manage Space->Clear All Data

2.   Please make sure the mobile is not running out of space.

3.   After clearing the data, open the proctortrack app again and wait for a few seconds. It will download the 

     QR code ml model in the background and once the download completes, the camera preview will start.

4.  Please download the ML Kit Demo app from the play store to check if the QR code reader works using this 

     app. We are using the same API in our proctortrack app. 

ML Kit Demo app link -   

5.  Once it is installed, please choose 'Barcode Detection' from the drop-down menu and see if it works.