I think the fix here is simple - when using a nested for loop inside a parfor loop, the bounds of that loop must be known to be constant for the duration of the parfor loop (at least if you want to "slice" a variable inside). In this case, you simply need to calculate numel(am) outside the loop, like so:
n_am = numel(am);
parfor biases = 1:numel(biass)
for ampl = 1:n_am
heuristic_map(ampl, biases) = ...;
end
end
This limitation is described in the documentation - see the section entitled Limitations of Nested for-Loops.
