error(c,m,g) is a call to the function error with input variables c,m,g .But you have not declared the error as a function ,you declared it as an expression depending on variables c,m,g .Also,the variables have to be declared before the expression.
g = 9.81; % gravitational force
m = 68.1; % mass of parachutist
error=[];
for c = 0:0.5:25
error(end+1)= ( (40*c) / (m*g) )-( 1-exp( (-10*c)/m ) );
end
y=zeros([1 50]);
h1 = figure(1);
plot(c, error, 'r--', 'Linewidth', 3);
hold on;
plot(c,y,'k-', 'Linewidth', 3);
