You must unfreeze pathfinding edits to modify the terrain.

[Damianlj]

I can't editing terrain ! on my own percell

[Rolig Loon]

As far as I am aware, Pathfinding is still only available on a handful of regions on the main grid that have asked to be part of the beta project.  If you are on one of those sims and are having problems, you should talk with the sim owner or let Linden Lab's beta team know the nature of the problem so they can deal with it.  The sim owner will know who to contact.

[Ansariel Hiller]

Your parcel is on a region with pathfinding support. You will have to use the pathfinding development viewer to be able to edit terrain unless the pathfinding tools are released to the public and other viewers can pick it up: http://wiki.secondlife.com/wiki/Linden_Lab_Official:Alternate_Viewers#Pathfinding