Abdominal CT registration using Elastix

I’ve tried using scissors and surface cut as a mask (in fixed as well as moving volume mask), however I get these two errors:

‘Could not find enough image samples within reasonable time. Probably the mask is too small’
‘Too many samples map outside moving image buffer: 265 / 32768’

I’m not sure what I’m doing wrong considering I’ve done as what you suggested me to do.
If I’m right you tell me to do it in these steps: create mask --> save as labelmap --> use this mask in Elastix (fixed/moving volume mask) --> Save Transform --> Apply transform to non-clipped/masked volume. What I do not get is that Elastix seems not to accept my masks to apply for registration…