sslv3 alert certificate unknown

The way to make key file and crt file:

#generate ca key

openssl genrsa -out ca.key 2048

#generate ca crt

openssl req -new -x509 -days 36500 -key ca.key -out ca.crt


################################################

#generate server private key

openssl genrsa -out server.key 2048

openssl req -new -key server.key -out server.csr

openssl x509 -req -days 3650 -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt

the server code:

std::string cert_file = "server.crt";

std::string key_file  = "server.key";

boost::asio::ssl::context  ssl_ctx_(boost::asio::ssl::context::tlsv12);

ssl_ctx_.use_certificate_file(cert_file, boost::asio::ssl::context::pem);

ssl_ctx_.use_private_key_file(key_file, boost::asio::ssl::context::pem);

the web js code:

                let url = "wss://172.29.78.76:4443";

                console.log("websocket url:", url);

                try {

                    let wsConn = new WebSocket(url);

                } catch (error) {

                    console.log("websocket exception:", error);

                }

but server report error:
sslv3 alert certificate unknown。

please help.