You can't update the parfor reduction variable result inside the inner loop, you need to update it directly in the body of the parfor loop. You can work around this by making a new temporary reduction variable for each iteration of the parfor loop, and then update result once that's complete, like so:
result=[];
for m=1:10
parfor k=1:100
tmp = [];
for c=1:50
a=m+k*c;
b=m*k-c;
tmp=[tmp;a b];
end
result = [result; tmp];
end
end
