Hi @hamish_234, thanks for posting on the forum and welcome to the SimScale Community 
This message usually appears when users forget to delete solid bodies apart from the flow region when running single-region analysis types (e.g. Incompressible, Compressible, Convective Heat Transfer…). These are not concerned with solid-fluid interactions and you’ll only be able to simulate using a single flow volume.
Is this your case?
As a sidenote, always make sure to add your project’s link when posting - this will make the troubleshooting task smoother for us! For more information, take a look here: Guide to Clear and Understandable Forum Posting
Cheers,
Igor