How can I resolve iPhone 4 activation error?

Because the iPhone 4 uses the 3G cellular band which has been deactivated in the US. and most parts of the world.

The iPhone 4 is very obsolete, you probably won't find any apps in the App Store that even work with it, and the battery is most likely shot. It's time to retire that phone.

To the best of my knowledge it can no longer be activated, except in a few isolated regions/countries. The activation servers have all been shutdown in the vast majority of countries. Sorry.