This is a common issue, and typically means the drive is already protected by BitLocker. Please see below:
TECH231177
Drive Encryption does not start automatically: DE Error -12368
Last Updated June 28, 2019
https://www.symantec.com/docs/TECH231177
TECH235632
Troubleshooting: User Registration and Single Sign-on with Symantec Endpoint Encryption
Last Updated April 15, 2019
https://www.symantec.com/docs/TECH235632
Searching for -12368 on https://support.symantec.com/us/en.html got me exactly these results, only. I'd highly recommend searching there as a first step for future issues, as there's a large body of historical issues addressed in Symantec's KB Articles.