Getting an Error:(22, 20) Kotlin: Unresolved reference: string error when adding the string command

I'm getting this error Error:(22, 20) Kotlin: Unresolved reference: string when I add some code to my learnign program I made in Intellij It runs ok until I add the string command then I get the error above.I have installed jdk-11.0.3_windows-x64_bin with ideaIC-2019.1.3.exe but not sure if this is the correct SDK to use? Or if this is causin the issues?

var department:string?
department = null
department ="software Engineering"
println ("department ${department}")

Information:Kotlin: kotlinc-jvm 1.3.40 (JRE 11.0.3+12-LTS) Information:Module "Varibles" was fully rebuilt due to project configuration/dependencies changes

Information:03/07/2019 16:20 - Build completed with 1 error and 0 warnings in 2 s 534 ms C:\Users\lanca\IdeaProjects\Varibles\src\DataTypes.kt Error:Error:line (22)Kotlin: Unresolved reference: string