sbt submit gives "not a valid command" error

You need to be using the code package for the course downloaded from the Coursera course web site, otherwise there will be no submit command available as it's not a built-in of SBT or Scala or anything. This is also covered by the tutorial video lecture, and you will be able to obtain the link from the course site.

Also, you need to provide your e-mail address and submission key (which you can obtain from the course web site) as parameters to the submit command.