Filters & Transforming
Why it appears: The Add Noise filter (found under the Filter>Noise submenu) can only add to a layer that has pixels, so if you're on an empty layer or the selected area contains no pixels, you'll get this warning.
How to fix it: Ensure the layer has pixels, or fill the layer or selection with color by pressing Option-Delete (PC: Alt-Backspace) before applying the Add Noise filter.
Why it appears: You attempt to apply a filter to a Type layer, but Type layers must be rasterized before a filter can be applied.
How to fix it: Although you could click OK and rasterize the Type layer, I recommend clicking Cancel, duplicating the Type layer (by pressing Command-J [PC: Control-J]), and rasterizing the copy (by choosing Layer> Rasterize>Type). That way you have a backup plan.
How to prevent it from happening: Duplicate the Type layer before applying the filter, or use a clipping mask (see Chapter 4, “Creating Flexible Documents”).
Why it appears: If you're using Free Transform (Command-T [PC: Control-T]), almost every other operation will be unavailable. (Note: If you try to use most of the tools, you'll get this warning dialog.)
How to fix it: Click Apply to finish your transformation and use your current tool; click Don't Apply to exit Free Transform without transforming your object; or click Cancel to finish transforming before proceeding with your tool.
How to prevent it from happening: Make sure you press Enter to finish transforming before continuing. Note: In Photoshop CS2, you can change the layer blend mode, opacity, and fill of a layer while in Free Transform.