Postgres SSL client certificate - database server CA failed to start

I want to configure SSL so that clients can only require certificate signed by root CA

.crt

, then every client must use their issued e.g. cert **postgresql.crt**, key **postgresql.key** and **password** to connect to database server. Hereby allowing the database server to verify each client certificate on authentication. Postgres v14 server configs: **postgresql.conf**

...
ssl = on
ssl_ca_file = ca.crt   # root CA or (root+intermediate CA bundle) that signed client cert.
#ssl_cert_file =
#ssl_key_file =
...

**pg_hba.conf**

...
hostssl  all  all  all  md5  clientcert=verify-ca
...

Starting database server failed with this error.

...
FATAL: could not load server certificate file "server.crt": No such file or directory
...

Why does it fail, I've no need for **server.crt** and **server.key** because server should be doing the client verification using

.crt

not the client? Reading postgres document doesn't help me. Thanks in advance.