Why this precondition will be fail,I don't understand well

Question

datatype message = Nil | Aenc(aencMsg:message,aencKey:message) | Senc(m1:message,m2:message) | K(r1:role,r2:role) | Pk(r:role) | Sk(r:role) | Str(r:role) | Var(n:rol