Sorry, we only support the latest Houdini production builds available. As you could understand we can't release plugin builds for all the available Houdini production builds, and this one is too old, with at least three other 18.0 Houdini production versions released later. You are going to need to update Houdini to one of the versions supported by our plugin.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini