Why am I getting the 'Too many output arguments error'

Why am I getting the too many argument error for 'qi = Kp*error'?

clc;
clear;
close all;

%initializations go here
T = 0.1;
    a = 1;
    h = 1;
    tfinal = 99;
    Kp = 3;
for time = 0:T:tfinal
    k = 0;
    k = k+1;
    qo(k) = 0.4;
    qi=Kp*error; %proportional control law 
    if qi > 0.5, qi = 0.5; 
    end %saturation limit: upper value
    if qi < 0, qi = 0; 
    end %saturation limit: lower value
    h = h + (1/a)*T*( qi - qo(k)); %time evolution of the liquid level from the TF
    error = r(k)-h; %error
    tiempo(k) = time;
    level(k) = h;
    inflow(k) = qi;
end

figure(102)
plot(tiempo, inflow,'r')
ylabel({'$q_i(t)$'},'FontUnits','points','interpreter','latex','FontSize',13,'FontName','Times')
xlabel({'$time (s)$'},'FontUnits','points','interpreter','latex','FontSize',13,'FontName','Times')
title('Time evolution of the inflow rate')

figure(10)
plot(tiempo, level, 'r')
hold on
plot(tiempo, r, 'b')
hold on
ylabel({'$r(t)$'},'FontUnits','points','interpreter','latex','FontSize',13,'FontName','Times')
xlabel({'$time (s)$'},'FontUnits','points','interpreter','latex','FontSize',13,'FontName','Times')
title('Time evolution of the input and liquid level')

Answer 1

[dpb]

error is a Matlab-supplied function which returns no outputs; hence trying to assign to a LHS fails. See

doc error % for details on that usage.

While you could alias the builtin; that is not recommended; instead, use some other name for an error magnitude variable... err might be a good choice--succinct but hardly likely to be misunderstood as to what is intended.