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As mentioned in the installation docs, RenderMan for Blender requires RenderMan Pro Server (RPS) to be installed on your machine. How the addon finds RPS can be set in the preferences:
Changing RenderMan Location to Choose From Installed will allow you to select from the ones that are installed in the default location. You can also set it to Set Manually and manually set the location of RPS. The default is to consult the RMANTREE environment variable. Any changes to this will require a restart of Blender.
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Alternatively, you can also render to RenderMan's "it" framebuffer by changing the Render To setting from "Blender" to "it".
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