Many points to make here. First, DON'T use error as a variable name. In general, it is never a good idea to use variable names that are the same as existing function names. You will one day then have a problem, and ask franticly why your code does not work.
To compute the integral, yes, you can use a sum, which would be just rectangle rule. So clearly you would have needed to multiply by the width of the rectangle. Nor was there any need to add an abs at the end there, or even to use sum! Look at what you did:
error1=error*error';
Why did you do this, and then try to sum it up? That is a dot product. It multiplies the elements of the vector, then sums them. Had you just multiplied it by deltat, that would be the desired approximation to the integral already.
Next, learn to use MATLAB, as it is designed to be used. Instead of a loop to compute the error, just use vector operations. So one line suffices instead of the loop.
err = 1-y;
deltat = 0.1;
ise = trapz(deltat,err.^2);
Even shorter, you might have done it all in one line:
ise = trapz(0.1,(1-y).^2);
