It works for me with 5.21.1 + gcl + Windows; maybe you need 5.20.1 instead of 5.20.0?
Suggestion: Try defining your functions using something like:
(%i15) integrate(exp(-w^2/2),w)$
(%i16) distri0(w):= ''%$
(%i17) distri(q):=(1/sqrt(2*%pi)*(sqrt(%pi/2)-distri0(q)))$
In your code, if you later assign A a new value, the function distri0 will misbehave.
--Barton
-----maxima-bounces at math.utexas.edu wrote: -----
>A:integrate(exp(-w^2/2),w);
>distri0(q):=float(subst(q,w,A));
>distri(q):=float(1/sqrt(2*%pi)*(sqrt(%pi/2)-distri0(q)));
>But?then?on?Maxima?5.21.0?(the?official?binary?gcl?for?windows)?the?
>calculation
>distri(0);
>yields?the?error?message