Your code does not generate that error message, but in general, indexes in MATLAB start at 1 not 0 like in C and some other languages.
clc; % Clear the command window.
close all; % Close all figures.
clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
fontSize = 22;
k=10e6;
cmin=10e-9;
cmax=10e-6;
c(1)=100e-9;
v=1;
for t=1:6
c(t+1) = c(1)-sqrt(1+(2*c(1)^2*k*(1/cmin-1/cmax)*v*t));
end
t = 0 : 6;
plot(t, c , 'rd-', 'LineWidth', 2);
grid on;
xlabel('t', 'FontSize', fontSize);
ylabel('c', 'FontSize', fontSize);
