See this forum thread,
https://openbci.com/forum/index.php?p=/discussion/2507/wifi-connection-with-different-esp8266-resolved/
"...When I downloaded the ArduinoJson lib, the last version is 6.X. However, the source code of the WiFi shield available in Arduino format needs ArduinoJson lib version 5.X to compile. ..."
As a workaround he instead used v 5.x, to match the current Shield code.
See this forum thread,
https://openbci.com/forum/index.php?p=/discussion/2507/wifi-connection-with-different-esp8266-resolved/
"...When I downloaded the ArduinoJson lib, the last version is 6.X. However, the source code of the WiFi shield available in Arduino format needs ArduinoJson lib version 5.X to compile. ..."
As a workaround he instead used v 5.x, to match the current Shield code.