Hi.
That library is part of the ESP32 arduino core. So, it is included by default. You don’t need to install anything to start using the library. Just make sure you have an ESP32 selected in Tools > Board.
Regards,
Sara

No doubt about it, I asked a very silly question...
Of course you are right, but I was convinced to attack an esp 32 and instead it was an esp8266 ...
Thanks Sara !!!

The ESP32 and ESP8266 use different ESP-NOW libraries.
Make sure you use the right code/library for the board you’re using.
I’ll mark this issue as resolved. If you need further help, you just need to open a new question in our forum.
Regards,
Sara