Xcode 9 Swift Language Version (SWIFT_VERSION)
For Objective C Projects created using Xcode 8 and now opening in Xcode 9, it is showing the same error as mentioned in the question.
To fix that, Press the + button in Build Settings and select Add User-Defined Setting as shown in the image below
Then in the new row created add SWIFT_VERSION as key and 3.2 as value like below.
It will fix the error for objective c projects.
Answer to your question:
You can download Xcode 8.x from Apple Download Portal or Download Xcode 8.3.3 (or see: Where to download older version of Xcode), if you've premium developer account (apple id). You can install & work with both Xcode 9 and Xcode 8.x in single (mac) system. (Make sure you've Command Line Tools supporting both version of Xcode, to work with terminal (see: How to install 'Command Line Tool'))
Hint: How to migrate your code Xcode 9 compatible Swift versions (Swift 3.2 or 4)
Xcode 9 allows conversion/migration from Swift 3.0 to Swift 3.2/4.0 only. So if current version of Swift language of your project is below 3.0 then you must migrate your code in Swift 3 compatible version Using Xcode 8.x.
This is common error message that Xcode 9 shows if it identifies Swift language below 3.0, during migration.
Swift 3.2 is supported by Xcode 9 & Xcode 8 both.
Project ► (Select Your Project Target) ► Build Settings ► (Type 'swift' in Searchbar) Swift Compiler Language ► Swift Language Version ► Click on Language list to open it.
Convert your source code from Swift 2.0 to 3.2 using Xcode 8 and then continue with Xcode 9 (Swift 3.2 or 4).
For easier migration of your code, follow these steps: (it will help you to convert into latest version of swift supported by your Xcode Tool)
Xcode: Menus: Edit ▶ Covert ▶ To Current Swift Syntax
