Due to a server-side change from Apple a few month ago, Cydia Impactor became unavailable. A change in Apple's provisioning service a couple of weeks ago changed the authentication method for free Apple accounts i.e. users without a paid Apple Developer account. ... But now, Cydia Impactor is still not fixed.

What happened Cydia impactor?

About two weeks ago, Apple made a change to their provisioning service to require a different authentication scheme for "free" Apple accounts (they return an error that mentions upgrading to "Xcode 7.3"); this broke Cydia Impactor for users without a paid Apple Developer account.

Do you need Xcode for Cydia impactor?

You do not need Xcode installed to use Impactor (even for features such as signing IPA files). To download new versions, use "Check for Updates..." under the Impactor menu from inside of the application. Impactor will also occasionally prompt about new versions that come out.