I have an ipad device with 5.1 OS and am trying to run a 5.0 base SDK app on this device.
here’s my error:

thanks for any help.

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I believe from your screenshot that you are running Xcode 4.2 with the iOS 5.0 SDK, in order to deploy to devices running iOS 5.1 you will need to upgrade to the latest version of Xcode (4.3.2) which has the iOS 5.1 SDK and will allow you to deploy your build to a 5.1 device. This upgrade will require OS X 10.7 Lion. Even though you are targeting a version of iOS supported by your version of Xcode, you can not deploy to devices running a newer version of iOS than your SDK supports.
(Lion upgrade alternative https://stackoverflow.com/a/9621226/716216)