title: SBT run statement shows: No main class detected
body: <p>I have created a web app and when I try to run with <code>sbt run</code>, it shows: </p>
  
  <pre><code>sbt run    
  [warn] No sbt.version set in project/build.properties, base directory: /home/developer/scala/user-svc/target/scala-2.13
  [info] Loading global plugins from /home/developer/.sbt/1.0/plugins
  [info] Set current project to scala-2-13 (in build file:/home/developer/scala/user-svc/target/scala-2.13/)
  [error] java.lang.RuntimeException: No main class detected.
  [error]         at scala.sys.package$.error(package.scala:30)
  [error] stack trace is suppressed; run last Compile / bgRun for the full output
  [error] (Compile / bgRun) No main class detected.
  [error] Total time: 0 s, completed May 23, 2020, 11:44:32 PM  
  </code></pre>
  
  <p>The content of <code>build.sbt</code> is: </p>
  
  <pre><code>val Http4sVersion = "0.21.4"
  val CirceVersion = "0.13.0"
  val Specs2Version = "4.9.3"
  val LogbackVersion = "1.2.3"
  
  lazy val root = (project in file("."))
    .settings(
      organization := "io.example",
      name := "user-svc",
      version := "0.0.1-SNAPSHOT",
      scalaVersion := "2.13.2",
      libraryDependencies ++= Seq(
        "org.http4s" %% "http4s-jetty" % Http4sVersion,
        "org.http4s" %% "http4s-jetty-client" % Http4sVersion,
        "org.http4s" %% "http4s-circe" % Http4sVersion,
        "org.http4s" %% "http4s-dsl" % Http4sVersion,
        "io.circe" %% "circe-generic" % CirceVersion,
        "org.specs2" %% "specs2-core" % Specs2Version % "test",
        "ch.qos.logback" % "logback-classic" % LogbackVersion
      ),
      addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.10.3"),
      addCompilerPlugin("com.olegpy" %% "better-monadic-for" % "0.3.1"),
    )
  
  scalacOptions ++= Seq(
    "-deprecation",
    "-encoding", "UTF-8",
    "-language:higherKinds",
    "-language:postfixOps",
    "-feature",
    "-Xfatal-warnings",
  )
  </code></pre>
  
  <p>The folder structure should be correct:  </p>
  
  <p><a href="https://i.stack.imgur.com/eyuif.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eyuif.png" alt="enter image description here"></a>  </p>
  
  <p>What am I doing wrong?</p>