What is meant by this message in output? Where is the error exactly?
Thread::tdlen: Objects of unequal length in {y1,y2-y1^2 y5,<<5>>,36 y1^5 y3^3-72 y1^5 y2 y3 y4+48 y1^10 y4^2-36 y1^2 y2 y3^2 y5+72 y1^2 y2^2 y4 y5-24 y1^7 y3 y4 y5+30 y1^4 y3^2 y5^2-60 y1^4 y2 y4 y5^2+20 y1^6 y4 y5^3-5 y1^3 y3 y5^4+<<2>>}^{16,1,0,0,0,0,0} cannot be combined.
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int = Integrate[1/(x^3 - 1), x];
Map[Framed, int, Infinity]
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It means exactly what it says. The lists you are passing to Thread are not the same length.
Thread[f[{1, 2}]]
(* {f[1], f[2]} *)
Thread[f[{1, 2}, {3, 4}]]
(* {f[1, 3], f[2, 4]} *)
Thread[f[{1, 2}, 3]]
(* {f[1, 3], f[2, 3]} *)
Thread[f[{1, 2}, {3}]]
(* Thread::tdlen: Objects of unequal length in f[{1,2},{3}] cannot be combined. *)