Dear Melike,
Unfortunately your flair file was not correctly uploaded so I cannot run your project for the moment. You can try to upload it once more, but the behaviour you describe is expected.
In fact, I would suggest a completely different approach, already outlined in this older thread. Basically, instead of defining a 252Cf source, you should load the appropriate neutron or fission fragment (FF) disributions from 252Cf fission via source routine. This would completely circumvent the A<247 limit.
I do not know whether you wish to simulate only the fission neutrons, the FFs or both; there are plenty of experimental data to use either way. In the past I have personally also used the GEF code to generate realistic FF distributions to load in FLUKA via source routine, but they may not be relevant for your problem.
source_newgen.f has plenty of options for sampling the particle energy, including from arbitrary user-provided distributions, so setting up a neutron spectrum representative of 252Cf fission neutrons will not be difficult. Have a look at section 3.4.4 of the routine (Sampling from histogram), where a simple external file can be provided with the neutron energy distribution.
Try it out and let us know if there are still problems.
Best regards,
Andrea