PS CC - Straighten layer is disabled in ruler tool

I just drew a ruler in my drawing and want to straighten the drawing, but the Straighten Layer button is disabled.  What would cause this?

PS CC 2017.0.1.  I'm working with a 200 GB PSD file (11230 x 4820 pixels, 200 Pixels/Inch), RGB Color mode, 8 Bits/Channel.  I have only one layer (Channel Mixer) and the background, which is the original drawing.  I've tried using the tool in both the layer and the background and the button is disabled in both.

What should I look for?

Thanks,

Ken