What’s wrong? Which reasons can cause this? Button inactive.
All 3D Layers deleted.
HD geometry exist. I don’t how to remove it. <-- May be this?
Yes, the HD geometry is definately the reason.
then, how to delete this HD geometry?
i have no need of it… 
Use Subtool Master and make a duplicate with the Mirror option. The duplicate will not have the HD Geometry.
You can also simply use Tool>Clone to make a copy of the model without mirroring. The copy won’t have HD.
Why are you deleting lower levels, though? This is destructive behavior that closes the door on later options such as creating displacement/normal maps or even making gross changes to the model’s shape. You don’t gain anything by deleting the lower levels and lose quite a lot.
What about when you want to delete a hidden part of the mesh? Don’t you have to abandon lower levels of subdivisions at at times to preserve the shape you want to remove? Or am I missing something?