There is currently a bug in the Arduino Core for ESP32 causing the following error:
error: redefinition of 'struct ArduinoJson::Internals::StringFuncs<const char*>'
The solution is to disable
PROGMEM support in ArduinoJson.
To do that, just add the following line at the top of your program:
#define ARDUINOJSON_ENABLE_PROGMEM 0
Then, you may have the following linker error undefined reference to