This can be made compatible with parfor by taking a few relatively simple steps. Firstly, parfor workers cannot produce on-screen graphics, so we need to change things to emit a result. In your case, this is not totally trivial since your primary result xmax is being assigned-to in a not-completely-uniform manner - you're assigning different numbers of elements on different loop iterations. Not only that, it appears not to be possible to predict up-front how many columns xmax needs.
Secondly, you need to make some minor changes to the loop iteration to be compatible with parfor, which requires consecutive integer loop iterates.
So, the major change is to have the loop write individual rows of results to a cell array I've called xmax_cell. Outside the parfor loop, it's trivial to convert this back to matrix form.
Putting all this together, we end up with this, which works correctly in R2019b as far as I can tell:
clc;
a = 0.2; b = 0.2;
crange = 1:0.05:90; % Range for parameter c
tspan = 0:0.1:500; % Time interval for solving Rossler system
% PARFOR loop outputs: a cell array of result rows ...
xmax_cell = cell(numel(crange), 1);
% ... and a track of the largest result row
maxNumCols = 0;
parfor k = 1:numel(crange)
c = crange(k);
f = @(t,x) [-x(2)-x(3); x(1)+a*x(2); b+x(3)*(x(1)-c)];
x0 = [1 1 0]; % initial condition for Rossler system
[t,x] = ode45(f,tspan,x0); % call ode() to solve Rossler system
count = find(t>100); % find all the t values which is >10
x = x(count,:);
j = 1;
n = length(x(:,1)); % find the length of vector x1(x in our problem)
this_xmax = [];
for i=2 : n-1
% check for the min value in 1st column of sol matrix
if (x(i-1,1)+eps) < x(i,1) && x(i,1) > (x(i+1,1)+eps)
this_xmax(j) = x(i,1);
j=j+1;
end
end
% Keep track of what's the maximum number of columns
maxNumCols = max(maxNumCols, numel(this_xmax));
% Store this row into the output cell array.
xmax_cell{k} = this_xmax;
end
% Fix up xmax - push each row into the resulting matrix.
xmax = NaN(numel(crange), maxNumCols);
for idx = 1:numel(crange)
this_max = xmax_cell{idx};
xmax(idx, 1:numel(this_max)) = this_max;
end
% Plot
plot(crange, xmax', 'k.', 'MarkerSize', 1)
xlabel('Bifuracation parameter c');
ylabel('x max');
title('Bifurcation diagram for c');