Cant upload code onto board

Other Hardware 3rd Party Boards
1

I tried out the Elegoo Smart robot car; when I try uploading the sketch onto the board (connected with a sensor board v5.0) it worked fine for the first two times, but then it kept saying the following (I tried replacing the USB cables and reassembling the whole circuit, but it was of no good):

Arduino: 1.6.9 (Mac OS X), Board: "Arduino/Genuino Uno"

Sketch uses 2,414 bytes (7%) of program storage space. Maximum is 32,256 bytes.
Global variables use 236 bytes (11%) of dynamic memory, leaving 1,812 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
Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

This report would have more information with
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option enabled in File -> Preferences.

PLEASE HELP

2

This symptom can have several reasons: wrong connection, incorrect port, missing bootloader etc. You have to try step by step. You replaced USB cable so you can exclude this case. This is good start.
You wrote that you have something connected. Disconnect all and try bare UNO. Arduino cannot communicate with PC if something is connected to 0,1 pins. These are used for Tx/Rx via USB also.