A few days ago, I introduced a bug into 3dNwarpApply when "improving" the error messages. The result was that it wouldn't work at all, and would fail with a message like
Can't copy nwarp dataset or
Can't extend nwarp dataset
I have now re-improved the error message checking, so that 3dNwarpApply doesn't crash
every time. The binaries with this fix are now available (version 16.1.15), and anyone using the nonlinear warping functionality in AFNI should update their binaries as soon as possible.
Sorry about this.
The best-laid schemes o' mice an' men gang aft agley.