Go to Tools > Board and ensure you have selected the correct Arduino board (e.g., Arduino Uno).
If you are using a board that does not support SoftwareSerial or need higher baud rates, consider the library, which is more accurate but uses fixed pins. To get the exact code for your project, let me know: how to download software serial library arduino
The library is tied to specific AVR platforms. If you have a different board selected (e.g., an ESP8266 or STM32), the standard AVR SoftwareSerial won't load automatically. Go to Tools > Board and ensure you
It is pre-installed as a "platform bundled library" with the Arduino IDE. If you have installed the Arduino IDE, you already have SoftwareSerial. consider the library