I was trying to convert syntax of old Swift project to new one as per few suggestion on Stack Overflow I tried converting it through Edit > Convert > To latest Synta
I just had this issue. It's due to path references. Have you been moving files around and/or copying folders? Check if .xcodeproj is inside or outside of the main project folder. Also, make sure that the project's file and directory structure are intact.
/yourFolderProjectName/ <-- contains your .xcodeproj file AND folder with Xcode project name
/yourProjectFolder <-- here are .swift, info.plist, assets folder, etc.
/yourProjectXcodeFile.xcodeproj