parallelize nested for loop

Question

A is a 2D array, n is the matrix size and we are dealing with a square matrix. threads are number of threads the user input

#pragma omp parallel for shared(A,n,k) private(i) schedule(static) num_threads(threads)
    for(k = 0; k < n - 1; ++k) {
        // for the vectoriser
        
        for(i = k + 1; i < n; i++) {
            A[i][k] /= A[k][k];
        }

        

        for(i = k + 1; i < n; i++) {
            long long int j;
            const double Aik = A[i][k];
            
            for(j = k + 1; j < n; j++) {
            A[i][j] -= Aik * A[k][j];
            }
        }
    }

i tried using collapse but failed the error it was showing was work-sharing region may not be closely nested inside of work-sharing, ‘loop’, ‘critical’, ‘ordered’, ‘master’, explicit ‘task’ or ‘task loop’ region.

After what i though was correct, the time increased as i executed the code with more threads.

I tired using collapse. this is the output:

26:17: error: collapsed loops not perfectly nested before ‘for’
   26 |                 for(i = k + 1; i < n; i++) {

This is LU-Decomposition