I'm working in the Shader Editor. I have already inserted Node Wrangler in Edit > Preferences, Add-ons.
I'm trying to add Viewer but I obtain an error: Cannot add node Viewer into node tree Shader Nodetree: Not a geometry node tree.
Someone know how can solve it?
What did you try to do? Copy the Viewer node from the Geometry Node Editor to the Shader Editor? That does not work. Nodes of different editors may have same or similar names (the Math, Vector Math and many other nodes for example which also do the same operations in both), but those are completely different node systems.
Generally speaking, you can only add any kind of node within the editor from the Add menu or copy from one nodetree to another nodetree of the same kind, for example from one material to another. But not from one kind of editor to another like Shader Editor to Compositor or Geometry Node Editor.
If you want to get a Viewer node in the Shader Editor: depending on which version you are, there is no special viewer node anymore. Because the previewing of a node output now simply works by directly connecting the node to the Material Output, and that is what the shortcut Ctrl+Shift+LMB of the Node Wrangler nowadays does.
In the past there was an explicit Viewer node plugged inbetween, but under the hood this was basically an Emission shader which used the node's output which you wanted to preview as color with an emission strength of 1. Quite a long time ago now the Shader Editor worked now the same way as if there were an Emission shader between when you just plugged anything in it which was not a specific surface or volumetric shader, so there is no need for a dedicated Viewer node anymore.
