I'm using Swift 2.3 on Xcode 7.3.1 and when in AppDelegate I'm trying to import
SWIFT_VERSION = 2.3
You can't simply mix
swift3 in a single project within single target and this is also not a recommended approach.
The error which you are getting is due to
UserNotifications framework. This framework has been added in
xcode8 so you can't use this framework in
Also if you are planning to use swift2.3 in Xcode 8 then you have to use
Legacy Swift Version if you made it yes then you will be able to run your project which is in swift2.3, but you will be not able to use syntax of swift3.