can we not upload to arduino Uno when Ethernet shield is plugged in ?

I recently bought two ethernet shield and with both of these plugged into arduino uno , if I upload code to UNO via USB connected to my MacBook I get this message

Sketch uses 5,674 bytes (17%) of program storage space. Maximum is 32,256 bytes.
Global variables use 399 bytes (19%) of dynamic memory, leaving 1,649 bytes for local variables. Maximum is 2,048 bytes.
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 2 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 3 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 4 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 5 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 6 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 7 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 8 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 9 of 10: not in sync: resp=0x00
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x00

is it that I cannot upload to arduino UNO if ethernet shield is plugged into UNO ?

Did you try to upload WITHOUT the shield and then unplug the UNO and replace the shield and then power up ?

Some shields seem to use pins that the Arduinos need for USB.

yes I am able to upload code if shield is unplugged , but after uploading with shield unplugged when I put the ethernet shield back opening serial monitor shows nothing of the sketch I uploaded.

I have bought 2 ethernet shield none of them work, So I am not sure if I can even get a working ethernet shield online.

Would you like to post your skecth but if you do then PLEASE use code tags ( </> )

So many good people on here if its an issue in the code I am sure they will spot it.

Also some shields have quite short pins and may contact the USB connector. I put insulation tape over the top of the connector just in case it could be a problem.

I am using th web server example,

#include <SPI.h>
#include <Ethernet.h>

// Enter a MAC address and IP address for your controller below.
// The IP address will be dependent on your local network:
//byte mac[] = {
//  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED
//};
byte mac[] = {
  0x41, 0x6d, 0x8e, 0x52, 0xb7, 0xfc
};
IPAddress ip(192, 168, 1, 177);

// Initialize the Ethernet server library
// with the IP address and port you want to use
// (port 80 is default for HTTP):
EthernetServer server(80);

void setup() {
  Serial.println("Testing");
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }


  // start the Ethernet connection and the server:
  Ethernet.begin(mac, ip);
  server.begin();
  Serial.print("server is at ");
  Serial.println(Ethernet.localIP());
}


void loop() {
  // listen for incoming clients
  EthernetClient client = server.available();
  if (client) {
    Serial.println("new client");
    // an http request ends with a blank line
    boolean currentLineIsBlank = true;
    while (client.connected()) {
      if (client.available()) {
        char c = client.read();
        Serial.write(c);
        // if you've gotten to the end of the line (received a newline
        // character) and the line is blank, the http request has ended,
        // so you can send a reply
        if (c == '\n' && currentLineIsBlank) {
          // send a standard http response header
          client.println("HTTP/1.1 200 OK");
          client.println("Content-Type: text/html");
          client.println("Connection: close");  // the connection will be closed after completion of the response
          client.println("Refresh: 5");  // refresh the page automatically every 5 sec
          client.println();
          client.println("<!DOCTYPE HTML>");
          client.println("<html>");
          // output the value of each analog input pin
          for (int analogChannel = 0; analogChannel < 6; analogChannel++) {
            int sensorReading = analogRead(analogChannel);
            client.print("analog input ");
            client.print(analogChannel);
            client.print(" is ");
            client.print(sensorReading);
            client.println("
");
          }
          client.println("</html>");
          break;
        }
        if (c == '\n') {
          // you're starting a new line
          currentLineIsBlank = true;
        } else if (c != '\r') {
          // you've gotten a character on the current line
          currentLineIsBlank = false;
        }
      }
    }
    // give the web browser time to receive the data
    delay(1);
    // close the connection:
    client.stop();
    Serial.println("client disconnected");
  }
}

when I connect power to arduino with Ethernet shield plugged in only power LED lights up, and when I connect the ethernet cable to jack on ethernet shield no LED turns on on the ethernet jack

Consider removing the while serial statement

while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

Not saying it will fix it but certainly worth a try.