Please advice on the sudden rise of this error;
" Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later."
uwsgi not running maybe? We can just guess, but you have the log files to check… 
Kindly share location of log files;
/opt/indico/log indico.log ?
How could something working so well, throw an error as;
“(psycopg2.errors.UndefinedColumn) column categories.event_creation_mode does not exist”
Check /opt/indico/log and possibly your webserver’s log files in /var/log.
PS: As an admin managing a server and a webapp, you should be familiar with the default location for log files, and a simple ls /opt/indico would have revealed the indico log path
You could just assist without being cheeky or low key insulting you know. Good people are gettng rare these days sheesh
I was asking by the way.
Sorry, but the last few days we had a TON of questions where people showed no effort 
You edited your post and that question wasn’t there when I answered it 
That error sounds a lot like you didn’t do indico db upgrade and thus the database structure is out of date. Running that command should fix it.
I did upgrade previously.
restarted uwsgi working fine now. Thanks
That’s just the way it is. Some are beginners and some masters. Even some masters, have to begin on new things. Such is life. Soon you would also be a baby somewhere