Dear riya,
the issue in this case is with the post processing tool detsuw. It is limited to a maximum of 4096 channels per detector.
You can manually increase the limit by editing the file:
/usr/local/fluka/src/tools/detsuw.f
You need to increase the value of the NBNMX parameter on line 20.
After editing, you need to recompile the tools. First change to the directory:
/usr/local/fluka/scr
The use the following command:
sudo make
Afterwards the processing of the results should work without any issue.
There is no need to change anything in usreou.f routine. It only changes the energy value before the channel is selected.
Cheers,
David