Thank you Nicolas Roduit for your response.
I have followed the steps you suggested. And now I am at following point:
2020-08-15 09:37:47,359 DEBUG [org.keycloak.adapters.AuthenticatedActionsHandler] (default task-17) Policy enforcement is disabled.
2020-08-15 09:37:47,360 DEBUG [io.undertow.request.security] (default task-17) Authenticated as 3d2abc0d-1ac9-4693-abce-09af6398d5c6, roles [ADMINISTRATOR, auditlog, admin, user]
2020-08-15 09:37:55,649 DEBUG [io.undertow.request] (default I/O-1) UT005013: An IOException occurred: javax.net.ssl.SSLHandshakeException: Received fatal alert: certificate_unknown
at java.base/sun.security.ssl.Alert.createSSLException(Alert.java:131)
at java.base/sun.security.ssl.Alert.createSSLException(Alert.java:117)
at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:313)
at java.base/sun.security.ssl.Alert$AlertConsumer.consume(Alert.java:293)
at java.base/sun.security.ssl.TransportContext.dispatch(TransportContext.java:186)
at java.base/sun.security.ssl.SSLTransport.decode(SSLTransport.java:164)
at java.base/sun.security.ssl.SSLEngineImpl.decode(SSLEngineImpl.java:668)
at java.base/sun.security.ssl.SSLEngineImpl.readRecord(SSLEngineImpl.java:623)
at java.base/sun.security.ssl.SSLEngineImpl.unwrap(SSLEngineImpl.java:441)
at java.base/sun.security.ssl.SSLEngineImpl.unwrap(SSLEngineImpl.java:420)
at java.base/javax.net.ssl.SSLEngine.unwrap(SSLEngine.java:634)
at io.under...@2.1.3.Final//io.undertow.server.protocol.http.ALPNLimitingSSLEngine.unwrap(ALPNLimitingSSLEngine.java:75)
at io.under...@2.1.3.Final//io.undertow.protocols.ssl.SslConduit.doUnwrap(SslConduit.java:773)
at io.under...@2.1.3.Final//io.undertow.protocols.ssl.SslConduit.doHandshake(SslConduit.java:664)
at io.under...@2.1.3.Final//io.undertow.protocols.ssl.SslConduit.access$900(SslConduit.java:68)
at io.under...@2.1.3.Final//io.undertow.protocols.ssl.SslConduit$SslReadReadyHandler.readReady(SslConduit.java:1172)
at org.jboss...@3.8.1.Final//org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:89)
at org.jboss...@3.8.1.Final//org.xnio.nio.WorkerThread.run(WorkerThread.java:591)