ArcGIS Pro 2.9.5; color JPEG
I had a scenario where I forgot to populate the Raster parameter picklist:

So I got a "Failed to apply Grayscale" error.

After some fumbling around, I see now that the error message in the History pane lists the cause of the problem:

I'm wondering if the full error message could be displayed in the Raster Function pane, where I hit "Create new layer". Otherwise, the "Failed to apply Grayscale" error is too generic to be useful.
I'm a novice when it comes to rasters, so let me know if I missed something.
Related: