please in future, post your code into the post. It makes the life of others easier than having to read it into a figure.
In your case, the reason why it is not plotting a line is pretty simple. Your vectors "t" and "thrust_force" are in fact scalars because the for loop uses t as an "index" and you erase thrust_force variable at each iteration.
I would recommand you to write your loop like that:
t = 0:80;
thrust_force = NaN(size(t));
for ii = 1:numel(t)
thrust_force(ii) = tug_thrust(t(ii));