Why is the Edit>Define Brush command grayed out when I try to create a brush? What is going on?
This is what is going on.
When you define a brush, the size of the area being used to create the brush (a selection within the canvas, the whole canvas, etc.) cannot be larger than 2500px in any direction. If you go over this limit, the Define Brush command will be grayed out.
Elements 7,10,11,12,13,14,15 - Photoshop CC 2017 - Lightroom CC 2015 Windows 7 Pro & Windows 10 Pro