The most notable issue I can see is the un-supported character in the 8th line before the fix() call, which appears as a box.
Remove that character.
The code (turned into script from function and using the values present in the comments) runs without an error after removing that character -
%function [p, y]=comppoisson(tsim, lambda, mu,sigma, nrep, delta)
%simulate poisson process trajectories,normal jumps starting at zero
%output y: trajectories
% inputs: intensity parametrer, nosimulations, simulation time,
% delta:simulation interval
% mu, sigma: parameters jumps
tsim=1; lambda=5; mu=0.01; sigma=0.20; nrep=3; delta=1/365;
nint=fix(tsim/delta); %nro of intervals
NEV=zeros(nrep,nint);
CNEV=zeros(nrep,nint);
N=poissrnd(lambda*tsim,1,nrep);
y=zeros(nrep,nint);
p=zeros(nrep,nint);
ax=0:delta:1;
for k=1:nrep
JT=sort(tsim*rand(1,N(k)));
for j=1:nint
NEV(k,j)=length(JT((JT<j*delta)& (JT>=(j-1)*delta)));
if NEV(k,j)==0
CNEV(k,j)=0;
else
CNEV(k,j)=sum(normrnd(mu,sigma,1,NEV(k,j)));
end
end
p(k,:)=cumsum(NEV(k,:));
y(k,:)=cumsum(CNEV(k,:));
end
% plot(ax, [0 y])
%end
disp('The code ran without any error')
