It might be due to your graphics not meeting ANSYS Discovery Live Graphics Requirements.
- Make sure your GPU meets the recommended hardware requirements (8GB GPU RAM)
- Try running with a higher fidelity setting and/or with a larger GPU.
- Try to reduce the simulation bounds by removing some parts of the model.
- Last resort: Try increasing the feature size by thickening the walls, etc.
Also, check out below post on Discovery Live – “Thin” geometric features and relative GPU adjustments
Discovery Live: “Thin” geometric features and relative GPU adjustments
Regarding Inventor 2018 file, this version is currently not supported.