CM 6.3 officially supports openjdk 11, but I think there is a bug in the init scrip when it tries to find the java version, it fails. Looking at the script /opt/cloudera/cm/bin/cm-server, it looks like it tries to find the Java version, but it doesn't have a search path for openjdk 11.
Thanks for reporting this. Can you share a bit more regarding what you were doing, where the failure occurred, and some of how you arrived at your possible cause. I'm about to take a look at this on CentOS... what OS are you on and where is your jdk located?
CM will try to locate the SDK but in this case, it may need to be told explicitly.
To configure JAVA_HOME for Cloudera Manager, add it to /etc/default/cloudera-scm-server
After you have updated the file, restart Cloudera Manager with service cloudera-scm-server restart.