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The issue is that is_distributive is not defined for all posets, just for lattices -- that is, on the Sage data type of Lattice, not Poset. So one option would be to find all lattices and then filter those:

sage: n = 6
sage: posets = [p for p in Posets(n) if p.is_connected() and p.is_lattice()]
sage: lattices = [p for p in posets if LatticePoset(p).is_distributive()]


Here LatticePoset(p) converts the poset p explicitly into a lattice. You could instead use this at the end

sage: lattices = [LatticePoset(p) for p in posets if LatticePoset(p).is_distributive()]


so that the elements of lattices are lattices, not just posets. Or do the conversion first:

sage: n = 6
sage: lattices = [LatticePoset(p) for p in Posets(n) if p.is_connected() and p.is_lattice()]
sage: distributive_lattices = [p for p in lattices if p.is_distributive()]