Wifi rssi converting char[] to uint8_t
The argument passed to RSSI() must be an integer indicating the array index of the SSID of the WiFi network whose RSSI you want to get. To get that integer, you must first scan for networks.

char ssid[] = "greenNet";
byte num_ssids = 0;
long rssi = 0;

num_ssids = WiFi.scanNetworks();
for (byte net = 0; net &lt; num_ssids; net++){
    rssi = WiFi.RSSI(net);
    Serial.print(WiFi.SSID(net)); Serial.print(": ");
    Serial.println(rssi);
}