You get the error because dpdt is empty, and thus dydt is 2x1.
Instead of defining kpm as global, it is better to define it as an input to the ode function. MathWorks recommends to avoid the use of global as well
Note that in your code t and y are being overwritten in each iteration of the loop. I have modified t and y to store the result of each iteration
%Defining constant value variables outside the loop
kpm = 27:1:30;
tspan = 0:30;
yo=[0.01 0.05 0.179];
n = numel(kpm);
nt = numel(tspan);
nyo = numel(yo);
%Pre-allocation
t = zeros(nt,n);
y = zeros(nt,nyo,n);
%Ode Loop
for k = 1:n
[t(:,k),y(:,:,k)]=ode45(@(t,y) newwaykumar2004(t,y,kpm(k)), tspan, yo);
end
%t(:,k) represents a solution corresponding to kpm(k)
t
%y(:,:,k) represents a solution corresponding to kpm(k)
y
function dydt=newwaykumar2004(t,y,kpm)
kp=3;
kmp = 25;
klm = 15;
kl = 1;
theta=1;
w=0.5;
%function handle
f = @(m) 1+tanh((m-theta)/w);
dpdt=kp*y(1)*(1-y(1))-kpm*y(1)*y(2);
dmdt=(kmp*y(1)+y(3))*y(2)*(1-y(2))-y(2);
dIdt=klm*f(y(2))-kl*y(3);
dydt=[dpdt;dmdt;dIdt];
end
