`no main class detected` on scala 2.13.3

[drewboardman]

I've scaffolded an example project using the template. This causes sbt run to fail with

[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 Oct 19, 2020 9:53:05 AM

[drewboardman]

user error