I see several things that could be the issue.
Try doing
clear all
Before you run your script. Then you'll notice that n is not defined.
Secondly, vc is a vector, so when you write
if vc==vi
you are comparing a vector to a scalar.
Another thing is that vc==vi will probably never be true. What you want, I assume, is
if vc>=vi
I am not entirely sure what you want: do you want to increase vc by the amount
((va*s)/(s+((d-s)/er)))
for every time step, resetting it to zero and increase n by one when it's equal to or greater than some threshold? If so, the code below should do that.
if true
% code
end
n=0;
er =5;
d=1.2*10^-2;
promot=('enter the peak voltage\n');
vm=input (promot);
promot=('enter the inception voltage\n');
vi=input (promot);
promot=('enter void thickness\n');
s=input(promot);
vc=0;
for t=0 :.1:3.14
va = vm *sin(2*t);
vc=vc+((va*s)/(s+((d-s)/er))); %increase vc by the amount ((va*s)/(s+((d-s)/er)))
if vc>=vi
vc=0;
n=n+1;
else if vc<=-vi
vc=0;
n=n+1;
end
end
end
disp(n);
