Intervals.icu maps many “standard” fields from the fit file records to different names internally. Some of this is to match the names Strava uses since that is how this project started.
You can see the names here:
https://forum.intervals.icu/t/server-side-data-model-for-scripts/25781#ActivityStream
So it’s velocity_smooth instead of speed and so on.
Stream names that start with a Capital letter are for custom streams which may or may not exist. That is why you don’t get an error for “Speed” but do for “speed” (no such standard stream).