Arduino: 1.8.19 (Windows 10), Board: "NodeMCU 1.0 (ESP-12E Module), 80 MHz, 4M (1M SPIFFS), v2 Lower Memory, Disabled, None, Only Sketch, 115200"
esp_now_testing:3:21: fatal error: esp_now.h: No such file or directory
#include <esp_now.h>
^
compilation terminated.
exit status 1
esp_now.h: No such file or directory
This report would have more information with
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Couldnt compile for a node mcu esp8266 board. Have installed esp8266 library from boards manager 
#include <ESP8266WiFi.h>
#include <esp_now.h>
// Variables for test data
int int_value;
float float_value;
bool bool_value = true;
// MAC Address of responder - edit as required
uint8_t broadcastAddress[] = {0x24, 0x6F, 0x28, 0x7A, 0xAE, 0x7C};
// Define a data structure
typedef struct struct_message {
char a[32];
int b;
float c;
bool d;
} struct_message;
// Create a structured object
struct_message myData;
// Peer info
esp_now_peer_info_t peerInfo;
// Callback function called when data is sent
void OnDataSent(const uint8_t *mac_addr, esp_now_send_status_t status) {
Serial.print("\r\nLast Packet Send Status:\t");
Serial.println(status == ESP_NOW_SEND_SUCCESS ? "Delivery Success" : "Delivery Fail");
}
void setup() {
// Set up Serial Monitor
Serial.begin(115200);
// Set ESP32 as a Wi-Fi Station
WiFi.mode(WIFI_STA);
// Initilize ESP-NOW
if (esp_now_init() != ESP_OK) {
Serial.println("Error initializing ESP-NOW");
return;
}
// Register the send callback
esp_now_register_send_cb(OnDataSent);
// Register peer
memcpy(peerInfo.peer_addr, broadcastAddress, 6);
peerInfo.channel = 0;
peerInfo.encrypt = false;
// Add peer
if (esp_now_add_peer(&peerInfo) != ESP_OK){
Serial.println("Failed to add peer");
return;
}
}
void loop() {
// Create test data
// Generate a random integer
int_value = random(1,20);
// Use integer to make a new float
float_value = 1.3 * int_value;
// Invert the boolean value
bool_value = !bool_value;
// Format structured data
strcpy(myData.a, "Welcome");
myData.b = int_value;
myData.c = float_value;
myData.d = bool_value;
// Send message via ESP-NOW
esp_err_t result = esp_now_send(broadcastAddress, (uint8_t *) &myData, sizeof(myData));
if (result == ESP_OK) {
Serial.println("Sending confirmed");
}
else {
Serial.println("Sending error");
}
delay(2000);
}
<ESP8266WiFi.h>, the rest of this code seems to be for an ESP32 and not an ESP8266/NodeMCU.