15: Target error from Commit Flash write
This error is typically reported when the debugger is attempting to write to the flash, and the verify fails.
The most common cause for this error is insufficient power being supplied to the target while programming the flash. Flash programming increases the power consumption of the chip dramatically, and if your supply is not up to it, the flash programming will fail.
A common cause of insufficient power is PC USB ports, which are not supplying the USB spec of 500mA. Try another USB port on the PC, or use an external power supply to provide the power instead.