OK. A couple of things to try. Go into your Finder home file/library/preferences and find the document com.apple.iTunes.plist. Note there may be more than 1 of these. Choose the one as written here, the others will have suffixes after .plist. Put it in the trash can and start iTunes. This was your preference file. Starting iTunes will now recreate a fresh new one. You will have to enter your preferences again. Open iTunes preferences window to complete. Try your purchase again. It should ask you to enter your user name and password . This may clear your problem. Check in the app Keychain Access in your Utilities folder to make sure you are entering the correct detail.