Juraj:
Arduino Uno WiFi is a board with integrated esp8266, not an Uno with WiFi shield.
Even if I compile for Arduino/Genuino Uno instead of Arduino Uno WiFi board, the same error shows up.
pert:
I'm assuming you're talking about the WiFiClientSecure.h that's part of the ESP8266WiFi library bundled with the ESP8266 core for Arduino.That library is written for directly programming the ESP8266. The microcontroller you're attempting to compile for is the ATmega328P. The ESP8266WiFi library is not compatible with the ATmega328P so you can't do this.
What kind of library should I use then? Are there special libraries for ATmega328P / where can I find them?
pert:
Which WiFi shield?
Wifi R3 I think.
Thank you!