Automatic Weight fails totally I have this model here and it is symmetrical.
I removed doubles, I recalculated the normals, I fixed non-manifolds, I removed existing vertex groups. But still, when I try to parent my mesh to the rig it fails. What could be the issue here?
My .blend file
It looks like parenting high-poly meshes fails, I don't know why, but if you merge by distance with a value of 0.0005 it won't be noticeable and it will parent correctly (and anyway you should work with a much less polygon version)