Hi
Congrats you just found a bug, we will fix that in the next release in the meantime
Just copy this folder
C:\OttoBlockly\compilation\arduino\packages\arduino\hardware\avr\1.8.3\libraries\SerialCommand
Into this folder
C:\OttoBlockly\compilation\arduino\packages\arduino\hardware\megaavr\1.8.6\libraries
and then it should work as you see here:
