Hey I have the same problem with my nodeMCU esp8266 board except doing whatyou did doesn't change anything, here's my error.
In file included from C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/ArduinoHttpClient.h:8:0,
from C:\Users\CLEMO\Documents\Arduino\SoilMoisture3.ino\SoilMoisture3.ino.ino:3:
C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/HttpClient.h: In member function 'virtual void HttpClient::flush()':
C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/HttpClient.h:310:50: error: return-statement with a value, in function returning 'void' [-fpermissive]
virtual void flush() { return iClient->flush(); };
^
C:\Users\CLEMO\Documents\Arduino\SoilMoisture3.ino\SoilMoisture3.ino.ino: At global scope:
SoilMoisture3.ino:21:56: error: cannot allocate an object of abstract type 'HttpClient'
HttpClient httpClient = HttpClient(client, server, port);
^
In file included from C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/ArduinoHttpClient.h:8:0,
from C:\Users\CLEMO\Documents\Arduino\SoilMoisture3.ino\SoilMoisture3.ino.ino:3:
C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/HttpClient.h:41:7: note: because the following virtual functions are pure within 'HttpClient':
class HttpClient : public Client
^
In file included from C:\Users\CLEMO\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0-beta3\libraries\ESP8266WiFi\src/WiFiClient.h:27:0,
from C:\Users\CLEMO\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0-beta3\libraries\ESP8266WiFi\src/ESP8266WiFi.h:39,
from C:\Users\CLEMO\Documents\Arduino\SoilMoisture3.ino\SoilMoisture3.ino.ino:2:
C:\Users\CLEMO\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0-beta3\cores\esp8266/Client.h:29:21: note: virtual int Client::connect(const IPAddress&, uint16_t)
virtual int connect(CONST IPAddress& ip, uint16_t port) =0;
^
C:\Users\CLEMO\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0-beta3\cores\esp8266/Client.h:37:22: note: virtual bool Client::flush(unsigned int)
virtual bool flush(unsigned int maxWaitMs = 0) = 0;
^
C:\Users\CLEMO\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.5.0-beta3\cores\esp8266/Client.h:38:22: note: virtual bool Client::stop(unsigned int)
virtual bool stop(unsigned int maxWaitMs = 0) = 0;
^
SoilMoisture3.ino:21:12: error: cannot declare variable 'httpClient' to be of abstract type 'HttpClient'
HttpClient httpClient = HttpClient(client, server, port);
^
In file included from C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/ArduinoHttpClient.h:8:0,
from C:\Users\CLEMO\Documents\Arduino\SoilMoisture3.ino\SoilMoisture3.ino.ino:3:
C:\Users\CLEMO\Documents\Arduino\libraries\ArduinoHttpClient\src/HttpClient.h:41:7: note: since type 'HttpClient' has pure virtual functions
class HttpClient : public Client
^
exit status 1
cannot allocate an object of abstract type 'HttpClient'