ImageMath is a small tool on the DTIPrep package and it needs to find it in order to apply the computed brain masks. It looks like it finds it correctly (assuming that you have indeed ImageMath under the path listed in that line in the QCreport), but cannot start it. Not sure what the issue is as this is not a message from ImageMath, but from the system. Maybe you do not execution privileges for ImageMath? 

If you look at the properties of that executable, you should see whether you can/are allowed to execute it or not.

Martin