You can try below solutions to run build in your device.
Restart your iOS device (If device is recently updated) and your Xcode.
Make Sure that in build settings "iOS Deployment Target" is <= the version of your iDevice.
Otherwise you can goto - Product > Destination, select your device which would be showing as ineligible device.You will be able to select your device and run your build.
I also came across that if you have the iOS 8.4 then your Xcode doesn't have the SDK for iOS 8.4 due to which devices show as ineligible. To solve this issue
Download Xcode 6.4