Try renaming any cities that use special characters in their name, that has been reported as an issue with transferring regions to offline mode. Claim any cities that are not claimed. If you have any mods installed, remove them. Temporarily disable antivirus and anti-spam software while you're trying to transfer the region. Make sure you set your Origin and SimCity icons to "Run As Administrator" (right-click, choose properties, compatibility tab). In Origin, right-click the SimCity icon and choose "Repair Game".
If you still have the problem, try running SimCityRecovery.exe - Default location is \Program Files (x86)\Origin Games\SimCity\SimCityRecovery\SimCityRecovery.exe Have patience, it can take a while to run.