Repro:

Repro project is available in the comments.

Download and import the repro project above using 2021.2.0b12
Open the Scene "Terrain_GPULM"
Navigate to the Lighting window and click "Generate Lighting"
Observe that the bake completes in a few seconds, using low settings such as these
Now open the Scene, "Terrain_Enlighten"
Observe that a Lighting Parameters asset has been assigned to the terrain. This is using the lowest possible indirect resolution that we allow
Click "Generate Lighting" once again

Observed:

Enlighten's precompute step does not progress beyond "Create Geometry" and the progress bar becomes unresponsive.

Expected:

Using very low indirect resolutions such as this, it is expected that the precompute step should be feasible. The terrain in the bug repro isn't really that large at 300x300 units, this doesn't feel unreasonable for very low resolution GI (0.01 texels per unit).

Notes:

Given Enlighten is in maintenance mode, I am unsure how much time we will invest here, but it is quite likely that users will want to use features such as the new TerrainToolbox to create detailed terrains and use these with Enlighten. In fact, on of the benefits we promote for Enlighten is time-of-day, and this implies an outdoor worlds, which will very likely use Unity's Terrain system.

With this in mind, it feels like maybe we should do something here to support such a major product feature? If we find that we absolutely cannot, then perhaps a warning should be shown in the UI and the bake should early-out?