in LWE instance $b=A^t s +e$, can we find an orthogonal basis of coefficient matrix A in polynomial time, let it be B.Multiply B to get $Bb=Be$ as term with s will vanish. Then solve for $e$. After getting $e$, subtract from $b$ and get the equation without error. Solve for $s$.
My doubt is, "Is finding orthogonal complement matrix difficult?" otherwise LWE may be broken?