JavaFX is/was part of the Oracle JDK, but is no longer bundled in Java 11 JDK, and OpenJDK always had it separately bundled. But the openjdk implementation OpenJFX it is now available via maven repositories (not sure if these work with Java 8).
As you want to use them from Scala, you may be interested in ScalaFX, which is a wrapper around JavaFX that allows more idiomatic Scala code. It works with JavaFX 8 and 11. Their Github Page includes the sbt snippets you need. If you don’t want to use ScalaFX, try the snippet for Java 11 without the scalafx dependency line.
As you say you want to create a Tetris game, maybe a game engine is more suitable. You actually can use any Java Library from Scala, so there is more choice than just JavaFX. I don’t have any experience in that area, but know of the existence of at least three game engines for Java, libGDX, LitEngine and JMonkeyEngine. If you want to do more game development, they may be worth a look.