I figured it out, the fix was:
%%
%For true stress/strain we need to create the reduction of area as necking
%occurs
% point at which necking occurs: stress = 312.8 Mpa
true_Load = M.Load*1000; %kN
load_location = 312.8*area/1000; %location in text file
load_location;
%Necking begins: line 2043
%Fracture: line 2724
%2724-2043 = 681 lines of data need to be recreated for stress
%now that we have the location of the UTS we can take that and begin the
%reduction from original area to fracture area
reduc_Area = linspace(area, frac_Area, 674);
reduc_Area';
%redcu_Area should have comperable data for the
%necking that occurs till fracture. We now need to make
%true stress by inputting the extrapolated values into a new
%stress file
true_Stress = stress;
% true_Stress(2043:2716,:) = true_Load(2043:2716,:);
% true_Stress(2043:2716,:) = true_Stress(2043:2716,:)/reduc_Area(1:);
for i = 1:673
reduced_Area = reduc_Area(:,i);
true_Stress(2043+i,:) = true_Load(2043+i,:)/reduced_Area;
end%We now have true stress
By incorporating the for loop I was able to go in and change the value. linspace was creating a 674 by 1 array and when I tried to divide the true_Stress by reduc_Area it wasn't allowing it.
The for loop allowed me to create a single variable to divide by.