After reboot (Debian) tomcat not starting: No JDK or JRE found

I followed the installation procedure on Debian as mentioned here
https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationViaAPT/

I also used the Nginx reverse proxy as advised…

It all worked fine, until I rebooted the server.

Can someone here guide me with troubleshooting this issue? Thanks

The syslog showed:

Sep 21 17:42:56 xwiki systemd[1]: Starting Apache Tomcat...
Sep 21 17:42:56 xwiki systemd[120634]: tomcat.service: Failed to locate executable /opt/tomcat/bin/startup.sh: No such file or directory
Sep 21 17:42:56 xwiki systemd[120634]: tomcat.service: Failed at step EXEC spawning /opt/tomcat/bin/startup.sh: No such file or directory
Sep 21 17:42:56 xwiki systemd[1]: tomcat.service: Control process exited, code=exited, status=203/EXEC
Sep 21 17:42:56 xwiki systemd[1]: tomcat.service: Failed with result 'exit-code'.
Sep 21 17:42:56 xwiki systemd[1]: Failed to start Apache Tomcat.

and a bit later it showed:

Sep 21 17:58:43 debian2 systemd[1]: Starting Apache Tomcat 9 Web Application Server...
Sep 21 17:58:43 debian2 systemd[1]: Started Apache Tomcat 9 Web Application Server.
Sep 21 17:58:43 debian2 tomcat9[14691]: No JDK or JRE found - Please set the JAVA_HOME variable or install the default-jdk package
Sep 21 17:58:43 debian2 systemd[1]: tomcat9.service: Main process exited, code=exited, status=1/FAILURE
Sep 21 17:58:43 debian2 systemd[1]: tomcat9.service: Failed with result 'exit-code'.