This step can be enabled in Setup > Settings > Options > Fill Block Model Gaps.
Option to fill voids in block models with a default material. This is useful where the current mining surface is higher than the clipped block model extents (such as backfill ramps).
To fill block model gaps:
Create a new gap filling preset, such as “backfill”.
Assign the material type, volumes, tonnes, and other block model parameters.
In Setup > Phase Details, toggle which phases use which preset.