How to Solve “We’re sorry, we are unable to continue with your activation…” Error
Fix 1 : Device Restoring
Try to restore / upgrade your device to the latest iOS (now it’s iOS 6.1). After restoring setup your iPhone as a New Phone using iTunes. Only then restore iPhone from your backup and pass activation steps with active Sim card inserted.
See Apple support article here on how to restore your iPhone.
If this doesn’t work move to Fix #2.
Fix 2: Hosts file reseting
If you have ever used software like TinyUmbrella to save SHSH blobs or to upgrade or downgrade your iOS firmware then your hosts file on your computer is pointing to Cydia (store developed by Saurik) activation server insted of Apple’s original one. So, please, just reset the hosts file on windows, and try to activate your iPhone. This time it should work fine. The location of hosts file for Windows 7 / XP / MacOS / Linux could be found here.
Fix 3: Working SIM
Make sure you put a SIM card into iPhone when activating. Do it with working SIM of the carrier to which your iPhone is locked. Try activation process after some time with working Sim on other computer. Also try to activate over WIFI, assuming you upgrade your device to the latest iOS 6.1.
Fix 4: DFU Mode + clean Restore
Or the last thing you can do is to make a restore on the other computer with active SIM card inserted. Also you can do a clean restore / upgrade putting your iPhone in DFU mode. Use this guide to enter DFU mode and start the process when iTunes will detect your iPhone in a recovery mode.
Fix 5: Apple Care
This solution may work not for all users unless you have real information about your iPhone. You should call Apple Care and describe your reactivation problem. After they will verify you are the owner of the handsets like S/N, IMEI #, purchase date, owner name etc. If you provide all needed information they will ask to restore your iPhone during 24 hours. This time the activation process should work perfectly.