Watcher slack alert not working

Team, we have 6.8.4 setup and we are moving to 7.4.2.
we have slack and email alerts configured. email alerts are working fine. whoever slack alerts are not working. attached is the error and elasticsearch.yml file. this took place after following the steps mentioned in 7.x upgrade wizard. getting below error

[2019-11-09T15:31:26,078][ERROR][o.e.x.w.n.s.SlackService ] [Hotdata-node-10.130.0.53] failed to execute slack api http request
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.ssl.Alerts.getSSLException(Alerts.java:192) ~[?:?]
at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1946) ~[?:?]
at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:316) ~[?:?]
at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:310) ~[?:?]
at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1639) ~[?:?]
at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:223) ~[?:?]
at sun.security.ssl.Handshaker.processLoop(Handshaker.java:1037) ~[?:?]
at sun.security.ssl.Handshaker.process_record(Handshaker.java:965) ~[?:?]
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1064) ~[?:?]
at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1367) ~[?:?]
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1395) ~[?:?]
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1379) ~[?:?]
at org.apache.http.conn.ssl.SSLConnectionSocketFactory.createLayeredSocket(SSLConnectionSocketFactory.java:394) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.conn.ssl.SSLConnectionSocketFactory.connectSocket(SSLConnectionSocketFactory.java:353) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:141) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:353) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:380) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:184) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:88) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:184) ~[httpclient-4.5.2.jar:4.5.2]
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82) ~[httpclient-4.5.2.jar:4.5.2]
at org.elasticsearch.xpack.watcher.common.http.HttpClient.lambda$execute$0(HttpClient.java:175) ~[?:?]
at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_222]
at org.elasticsearch.xpack.core.common.socket.SocketAccess.doPrivileged(SocketAccess.java:32) ~[x-pack-core-6.8.4.jar:6.8.4]
at org.elasticsearch.xpack.watcher.common.http.HttpClient.execute(HttpClient.java:175) ~[?:?]
at org.elasticsearch.xpack.watcher.notification.slack.SlackAccount.send(SlackAccount.java:115) ~[?:?]
at org.elasticsearch.xpack.watcher.notification.slack.SlackAccount.send(SlackAccount.java:69) ~[?:?]
at org.elasticsearch.xpack.watcher.actions.slack.ExecutableSlackAction.execute(ExecutableSlackAction.java:50) ~[?:?]
at org.elasticsearch.xpack.core.watcher.actions.ActionWrapper.execute(ActionWrapper.java:144) ~[x-pack-core-6.8.4.jar:6.8.4]
at org.elasticsearch.xpack.watcher.execution.ExecutionService.executeInner(ExecutionService.java:526) ~[x-pack-watcher-6.8.4.jar:6.8.4]
at org.elasticsearch.xpack.watcher.execution.ExecutionService.execute(ExecutionService.java:308) ~[x-pack-watcher-6.8.4.jar:6.8.4]
at org.elasticsearch.xpack.watcher.execution.ExecutionService.lambda$executeAsync$5(ExecutionService.java:414) ~[x-pack-watcher-6.8.4.jar:6.8.4]
at org.elasticsearch.xpack.watcher.execution.ExecutionService$WatchExecutionTask.run(ExecutionService.java:615) [x-pack-watcher-6.8.4.jar:6.8.4]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:681) [elasticsearch-6.8.4.jar:6.8.4]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_222]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_222]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_222]
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:397) ~[?:?]
at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:302) ~[?:?]
at sun.security.validator.Validator.validate(Validator.java:262) ~[?:?]
at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:330) ~[?:?]
at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:237) ~[?:?]
at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:132) ~[?:?]
at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1621) ~[?:?]
... 34 more

it got resolved. es was verifying certificate for slack as well, changed verification mode from certificate to none.
xpack.http.ssl.verification_mode: none