Using Port : usb Using Programmer : stk500v2avrdude: usbdev_open(): did not find any USB device "usb" (0x03eb:0x2104)avrdude done. Thank you.An error occurred while uploading the sketch
The reset of the target board is necessary during the upload process. When the chip comes out of reset, the booloader looks to the serial port to see if an upload is in progress. If not, the boot loader runs the sketch that is already there. If there is serial activity the boot loader loads the incoming sketch. Since you are using an Uno as a FTDI there is no DTR signal to generate an auto reset so the reset must be done manually.
why is that? there is no cap in the reset circuit that I'm aware of. and even with two 10k resistors in parallel you've still got 5k of pull up. which is more than enough to create stability in ordinary working mode.