Auto Bone-Weight fails if:

1. The Object has vertices in it that are very tightly spaced

(Change and apply scale for weighting. DAZ topology causes this problem in the genital portions. The usual workaround is to scale up the mesh by a factor of 10, apply, weight it and then scale it back down to 0.1 and apply again.)

2. The Object is made up of separate Meshes that intersect

(separate by parts and auto-weight these individually)

3. Inconsistent Normal direction

(Enable "Face Orientation" or Normal Display from the Overlay Menu to find the offending areas, then chose either Mesh > Normals > Recalculate Outside or Recalculate Inside to make the normals conformal again.)

4. The Armature has no deform bones in it.

(That is a bone property; select a bone, check the "Deform" box)

5. The Mesh has Duplicate Overlapping Vertices

(technically the same as 1, remove with "Merge by Distance". Be mindful that this may mess up your topology and weld things together that should stay open, like mouths or genitals.)

The sure-fire way to solve weight paint error is to weight it manually instead of using Auto Weights.