Hi Bryson,
Sorry for the late,
It seems that SPM cannot write the result matrix to file,
I guess it is still a problem that some version's SPM8 was
incompatible to GRETNA.
You can try to test spm_write_plane function to ensure it can accept
cell-style input for the first one, if not, try to update to the newest
version SPM8 which I use, too.
If it is still not work, we can try to find any other problem to make it
run.

Best,
Sandy