When I want to install iOS 5.1 Simulator in Xcode 4.5 then I receive a following error
You can see I have installed iOS 5.0 Simulator without problems. Do you know how can I resolve this issue?
Thank you.
You can try to go to https://developer.apple.com/downloads/index.action and download it manually.
I had same problem as you and I wasn't able to download Command Line Tools from XCode 4.5 with same error message you are getting. I noticed when I logged into Member Center on Apple Developer portal that I had some warning message which was saying that Apple has changed some License Agreement text. This License Agreement change should be accepted. Without that I wasn't even able to gain access to iOS 6 SDK beta part of iPhone Developer portal. After accepting this, I was able to download latest version of XCode 4.5 and do all updates from XCode (Command Line Tools and iOS 5.0 Simulator as well).
While the issue is not fixed by apple you can find a temporary solution here
Credit goes to jesper-christensen in this question.
Cheers Jesper and Dominik