Met an error: DBT-06103 in configuring enterprise manager express port 5500 during Oracle 19c database installation.
In fact, DBT-06103 means that OUI cannot learn the local hostname from any name resolution, especially from /etc/hosts. It alerts you to solve it.
Just add an IP mapping to /etc/hosts for hostname resolution.
[root@ora19c ~]# vi /etc/hosts
10.10.10.10 ora19c ora19c.example.com
Then we can proceed the installation. DBT-06103 is solved.
As you can see in the screen shot, Enterprise Manager Database Expression URL was shown as hostname with port number, that's why EM needs to know your hostname during database creation, either from /etc/hosts or DNS.
Further reading: How to Access Enterprise Manager Database Express 12c