serializeJson() accepts a Print&, i.e, a reference to a class that implements the Print interface.
WiFiClient does implement the Print interface, but serializeJson() cannot make a reference to it because ESP8266WebServer::client() returns a temporary. Indeed, C++ forbids references to temporary variables, only const reference to temporaries are allowed.
To fix this compilation error, you must extract a variable:
WiFiClient client = server.client();
serializeJson(doc, client);
The way I see it, ESP8266WebServer::client() should return a WiFiClient& instead of a WiFiClient, just as HTTPClient::getStream() does. I opened issue #7075 about that.
Anyway, even it this fixes the compilation, I'm not sure it will work as you intend because ESP8266WebServer may not allow writing directly to the WiFiClient.
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