Did you select "old bootloader"?
There is so many problems with IDE 2.XX , I refuse to use it.
Now I am totally confused!
and again the same problem 
Just try old bootloader.
OK, brings us back to this possibility:
This 'external' app can potentially also be Arduino IDE's own Serial Monitor. Close that before doing a firmware download, and see if it makes a difference.
Note also that your COM port may only be temporarily unavailable if you have an intermittent hardware problem. You can trace this by keeping the device manager visible on your monitor while trying to download firmware to the device and see if the COM port temporarily disappears and then pops up. If you enable sound on your computer/laptop, you'll generally also hear a little sound if a USB device is being connected/disconnected; if you hear that sound, you know something's up.
same error
the sound is ON in my laptop, now i replugged the USB and it does make sound of disconnect and connect so USB connection between laptop and arduino cant be problem here
FYI: TX led is keep on blinking. which means code is in run. even if i click reset button. nothing happens. tx led keeps on blinking indicating tranmission. though yellow led lights up when reset button is pressed
After you release the reset button, the code restarts and the Arduino will continue sending data over its UART connection.
Are you sure you're understanding what "Reset" does? Reset literally re-sets the controller. It's like turning the device off and immediately on again. Logically, your program restarts, and if you're only looking at the TX light, it may indeed look like nothing really happened, since the same behavior seems to continue. In reality, the reset did occur, though.
If you press the reset button *and hold it pressed * (do not release it), does the TX LED still keep blinking? Hold down reset for 10 seconds or so. No blinking LEDs should occur in that timeframe, since the controller will be held in reset, which makes it unable to do anything during that period.
Note also that the Reset button on your Nano is not wired in any way to the FTDI chip that makes the COM port appear in Windows. So pressing Reset, holding it etc. as such will do nothing to the COM port.
oh okay thank you for clarifying. I misunderstood the function of reset button then. when i saw that error, i thought maybe the older code is still in the arduino's memory and maybe reset button will remove it.
What USB chip is on the Nano ,FTDI or CH340?
FTDI CHIP is on arduino
hey...finally found the solutionn for uploading problem. the serial monitor was the problem. i have to close it to upload.
now please help me find issue in circuit or code.
Ok, so that was this:
And this problem:
...apparently boiled down to this:
Now for this:
...could you please tell us one more time what the remaining problem is? You mentioned this:
One hint I see in your code is this:
This is the condition that you use to switch on the pump. Is it intentional that:
1: total volume is never decreased anymore?
2: once the pump comes on, it stays on until you reset the Arduino or power down the circuit?
Can you tell us in clear wording what the system is supposed to do?
system is suppose to turn off the pump when 0.5l of water is passed through the flow sensor.
now it seems to be working. but i am not able to download this data plot. could you please tell me how can i get this data? is there any software or code that i can use so that this data get converted into excel file or become downloadable?
Great, good to hear!
The easiest way is to open the Serial Monitor (not plotter) and simply copy-paste the values into Excel.
I would not consider tge serial monitor an "external" app 
There is a known issue in IDE 2.x for upload problems with the combination of the FTDI driver and serial monitor; it was reported before and is on github.
I've encountered the problem when I was using IDE 2.x. I might be able to upload 10 times with the serial monitor open and after that it will fail.
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