The adopted libraries are pretty complete.
On the other hand, a <20 MeV neutron cannot generate 225Ac on a thorium target, being the physical threshold of this reaction channel above 30 MeV. At those energies (>20 MeV neutrons), FLUKA uses its own reaction cross section parameterization and reaction model.
While - as already pointed out - FLUKA is not designed to extract microscopic cross sections, ways to estimate the latter have been already discussed at length in this forum, including a thread initiated by yourself.
As for the 225Ac activity in a thorium target irradiated by a broad-energy neutron source, it can certainly be calculated by FLUKA.