Hello Lillian,
To begin with, what type of installation you did, What I mean, did you use the ng-tools (Apache2) or maybe the Nginx and finally the standalone manual drove. That will help determine where the problem might be coming from.
if all this has been determined, clear the logs in the catalina.out or if you used the ng-tools in the /opt/dhis2/logs there is a log file in there. clear the logs and start the instance and share fresh logs in zip and upload them here.
Then we would have identified the problem and it will be resolved in no time.
Kind regards
Moses