Make sure to have the Render license installed.

If running Houdini Indie or Houdini Apprentice, the render license will be listed as Renderer (NC) meaning non-commercial render.

If you are trying to render using the most recent version of Houdini, the render license must also be at the current version.

For example, if you are trying to render in a Houdini 18.0 session, Mantra will look for a version 18.0 Render license.

Houdini licenses are backwards compatible. This means that you can run older versions of Houdini using the most recent licensed version. For example, Houdini 18.0 or below will open with a version 18.0 license.

This is true for all Houdini products - FX, Core, Engine, Mantra, Indie, Apprentice