Check the code...your input in function is B and you are trying to initialize B1 using mB, at this line your function doesn't know what is mB; in the next line you are defining mB .So the error. You need to reverse the lines. First define mB and the use it.
function B1 = SortSignificanceFirst(B)
[mB, ~] = size(B);
B1 = zeros(mB,1);
SI = zeros(mB,2);
for b=1:mB
[mBB, ~] = size(B(b,1));
SI(b,:) = [mBB b];
end
SI_Sorted = sortrows(SI);
for b=1:mB
B1(b,1) = B(SI_Sorted(mB-b+1,2),1);
end
