Once, on Ubuntu 18.04.6 LTS, I updated the Zabbix server to version 6.0 LTS and the new version did not start.
When starting, an error was displayed:
service zabbix-server start
Job for zabbix-server.service failed because the service did not take the steps required by its unit configuration.
See "systemctl status zabbix-server.service" and "journalctl -xe" for details.
journalctl -xe
zabbix-server.service: Can't open PID file /run/zabbix/zabbix_server.pid (yet?) after start: No such file or directory
systemd[1]: zabbix-server.service: Failed with result 'protocol'.
systemd[1]: Failed to start Zabbix Server.
Since Zabbix server version 5.0 was working successfully before the update, there should be no problems with the rights and the configuration file /etc/zabbix/zabbix_server.conf.
Therefore, you definitely need to look at the log file /var/log/zabbix/zabbix_server.log, by the way, when you start zabbix-server after an update, you can see the process of updating the database, at this moment it is not advisable to stop zabbix-server, you need to wait.
In my case, the process did not reach the zabbix database update, and in /var/log/zabbix/zabbix_server.log there was a message that the current version of the MariaDB database server is not supported by Zabbix Server 6.0:
Starting Zabbix Server. Zabbix 6.0.25 (revision 1706b11e866).
...
using configuration file: /etc/zabbix/zabbix_server.conf
Unsupported DB! MariaDB version 100148 is older than minimum required 100200
Unable to start Zabbix server due to unsupported MariaDB database version (10.01.48).
Must be at least (10.05.00).
Use of supported database version is highly recommended.
Therefore, to solve this problem, you just need to update the MariaDB database server, in my case manually, and not from the Ubuntu repository.
See my other articles about Zabbix