I understand that you are trying to find the determinant of matrix B. But, The determinant is a scalar value that can be computed from the elements of a square matrix and encodes certain properties of the matrix. For a non-square matrix, since the number of rows and columns is different, the concept of a determinant does not apply.
Please take a look at this documentation for better understanding: https://www.mathworks.com/help/matlab/ref/det.html
I hope this helps.
