Carthage Build Failed

Check your application folder and Switch your Xcode version to older version with swift 3.

sudo xcode-select --switch /Applications/Xcode.app

TL;DR: Ensure you're using the latest Swift version, included with Xcode, and then perform carthage bootstrap --no-use-binaries --platform iOS --cache-builds

Long version:

The error you got from Xcode while building your project, Incompatible Swift version - framework was built with 3.1 and the local version is 4.0, is due to a download performed by Carthage to get a pre-built version of Realm Framework (this is a time-saver feature but sometimes there are version incompatibilities like this one).

This downloaded pre-built framework was compiled with a previous version of Swift (in this case the error is pointing v3.1 was used.) The solution for this error will be to perform the dependencies installation process using a different command:

carthage bootstrap --platform iOS --no-use-binaries

This will work for the project dependencies build to succeed. Anyway this might take a long time since it will build from scratch the Realm.framework and RealmSwift.framework from the core every time the command is executed. So it is possible to enable a local cache for the builds on Carthage. The dependencies can be installed with

carthage bootstrap --no-use-binaries --platform iOS --cache-builds

With this options Carthage will:

• bootstrap, so the versions in the Cartfile.resolved will be used, no updates.
• --no-use-binaries build the dependencies with no downloads of pre-built frameworks (avoiding the version incompatibility.)
• --platform iOS will build the dependencies for be used only on iOS, avoiding building frameworks for Apple TV or Mac OS in case the dependency supports it. This will cut a lot of time!
• --cache-builds will enable Carthage to store your built dependencies (and use it when re-needed) on a local cache, so even if you perform the command again it will just copy your stored framework, avoiding another time consuming build process.