The view must contain a flat or cylindrical surface on which you can place decals. You cannot place decals in 3D perspective views.
If no decals exist in the current project, the Decal Types dialog displays, and you need to create a decal type. See Creating a Decal Type.
The decal displays as a placeholder in all non-rendered views, as shown. When you move the mouse over the decal or select it, it displays as a rectangle with a cross through it. The detailed decal image is visible only in rendered images.
3D view showing placeholders for decals
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