Be careful, there are some fundamental misconceptions in the exchange above.
Both in @profftee97’s input and in @blefebvre MATERIAL cards, no single lead isotope is defined at all, rather for all of them the natural composition is taken by FLUKA! This is because the isotope selection is made by WHAT(6) (mass number - integer given as real, e.g. 208.0 - of the material, as in the manual) that you left blank (implicitly asking for natural composition). As for WHAT(2), leave it blank, as the manual clearly states (NOT to be filled). The respective atomic weights (that are different from the mass numbers) will be correctly set by FLUKA automatically. Moreover, please leave blank the material index too (again NOT to be filled in case of name-based input, see the manual).
Then, LOW-MAT cards do not apply to compound materials, since low energy neutron cross sections are defined for their elemental components.
Finally, the FLUKA library contains low energy neutron cross sections only for the natural lead composition and for 208Pb. So, as far as low energy neutron transport in your lead material is concerned, you have only these options, irrespective of your own defined isotope composition. If you care about nucleus production by 100 MeV protons on a (thin) lead target of specific composition, the aforementioned unavailability is not a critical showstopper, though.