I just checked the work of Progressive Lightmapper with RWT, and it works well.
Without any errors and warnings.

Most likely the problem is in your settings.
But what exactly is very difficult to say, because I do not see your project.

Try to generate terrains in a new scene or project, and send me the steps and settings needed to reproduce the problem.

I have a problem about run "Generate Lighting" in "Lighting Settings" :

After I generate roads & terrain by Real World Terrain , I try to run the "Generate Liighting" in "Lighting Settings" .
But I got more warring messages like below :
  Requested realtime/indirect resolution on mesh '' is too high, clamping to maximum system size (512x512). Lower the resolution
  setting.
and finally , I got Error Message
  Progressive Lightmapper: AddMaterial job skipped - out of memory.

Could you provide the rule about How to fix this issue by modify some item in the Settings of "Real World Terrain" ?

Thanks