After installing the Gmail App:

1. Access the Google Admin App console to check whether the CAS App has been enabled for all users:
   1. On the Google Admin console, go to Apps > Marketplace apps.

      

   2. Check the status of Trend Micro Cloud App Security. It should be "On" for everyone.

      

       

      If you receive an error when installing Trend Micro Cloud App Security from the market, you could open Chrome in Incognito Mode and install it again.
2. Check whether the provisioned user has CAS App installed by clicking on the Google Apps button. Trend Micro Cloud App Security should exist there.

   

3. Check the Google Admin page about the advanced G Suite API setting:
   1. On the Google Admin console, go to Security > Settings.

      

   2. Refer to this G Suite Admin help article to enable API access, then check the apps for Gmail.

      

4. Ensure Trend Micro Cloud App Security has permission for Gmail.

   

For reference, check Provisioning a Service Account for Gmail.