"I think there is an issue when I call n(s)"
You are right. When t == 1, s = t-1 == 0. And as you are using s as an index, it gives the error.
Indexing in MATLAB starts from 1 (as can be inferred from the error message).
The solution is to define the value of 1st element manually and start the for loop from t == 2.
If the value is 0, you can remove the assignment, as you have already assigned it to zero.
function Initialproblem(N,T,p)
n=zeros(T,1);
n(1) = value_of_starting_point;
for t = 2:T
s = t-1;
n(t) = N*exp(-p)+n(s)*(1-exp(-p));
end
fplot(0:1:T,n_t)
end
