I guess you do not have installed any linux-image package inside the
nfsroot. Try: 

grep linux-image /var/log/fai/fai-make-nfsroot.log

Since you have installed the Ubuntu version of the FAI packages, your
/etc/fai/NFSROOT may not work for a Debian nfsroot.

Please check the diff of your NFSROOT to this version:
https://github.com/faiproject/fai/blob/master/conf/NFSROOT

In Ubuntu the kernel package ist called linux-image...-generic in
Debian it's linux-image-amd64. I guess this is the problem you have.

-- 
regards Thomas

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