Q: I can't download ios 10.0.3 !! Why ?

Shutdown and reboot the phone

About the security content of iOS 10.0.3 - Apple Support

if the update does not show connect your phone to a computer and update via iTunes

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

if you still do not see it there could be several things happening: It's possible software may not yet be available in your location. The update might also be prevented if the phone has been altered in any way.