Why did my wWindows recovery tool end up with a failure on my Lumia 520?

[Windows Central Question]

windows recovery tool ended up with failure lumia 520

my lumia 520 have problem unable to find bootable option,so ii downloaded wundows device recovery tool and downloaded the 1.37GB file but while installing the device is automatically disconnet from pc and it it shows "Operation ended with failure,the software is not correctly signed or not signed for the device"