Dear @ArghyaC,
In the manual, you can find the list of materials for which cross sections are available, and unfortunately, 251Cf is not (yet) amongst them.
One way to by-pass your error from a technical point of view is to assign a dummy material from the FLUKA manual to 251Cf, but you should pay great attention in choosing an existing material that should ideally be very similar to 251Cf. However, even a difference of one nucleon can yield drastically different results.
The closest/heaviest element that is available is 243Am, which nevertheless has quite a different non-elastic cross section as per the TENDL suggestion below.
Let me know if this solution is acceptable on your side.
Best,
Daniel