It seems like you're missing a couple of = signs in your code. Try using:
if (on && level<20) {
wifiManager = (WifiManager) this.getSystemService(Context.WIFI_SERVICE);
wifiManager.setWifiEnabled(false);
} else {
wifiManager = (WifiManager) this.getSystemService(Context.WIFI_SERVICE);
wifiManager.setWifiEnabled(true);
}