That’s cool, unfortunately the feature’s schema isn’t happy with the deb https://dl.bintray.com/sbt/debian / source line (the quirky SUITE part and the missing COMPONENT part)
The sources.list man page describes how to represent single line source specifications in the newer deb822 format. From there, it should be pretty easy to convert to the YAML format Snapcraft expects.
In this case, the suite is / and there are no components, so you would want something like this: