Hello all. As Syan had answered, I agree with that assessment. Best that I can figure out from trial and error, Canvas can slice a single material (for simplicity think of it as a single colour) and will generate a gcode file only. This would be no different than PrusaSlicer with a single colour that generates the gcode you upload to the i3.
The Palette device only reads .mafx code, and that code is only produced when Canvas slices two or more materials (or colours).
If you slice a single colour model and attempt to “send to device”, only the gcode was produced, therefore there is no .mafx file to send to Palette, hence the error 500.
In essence, the Palette is best used for multi-colour models, and multi-material models (PLA with TPU supports type models).
If you want to just print a model with a single material, save the Canvas generated gcode to the SD card, and upload it to your i3.
I received my Palette 3 Pro last week. I spent a good day figuring out the Prusa Mk3s setup, then moved on to how to set up colour transitions values, and now I’m moving on to modelling for successful multi-colour prints.