I can't seem to figure out what's wrong with the code

I've attached my copy of the code..and included is the original code from Berkeley Madonna

Not 100% sure what I'm doing wrong, and I need to determine when SA will reach steady state. How do I go about that?

[Walter Roberson]

Total SA = (SA1 + SA2)/140;

You have a space in the variable name. However, that variable does not appear to be used for anything.

function ddt = odefun(t,y)
[...]
% Flows

JSA = k12 * SA1 - k21 * SA2;

You have not defined SA1 or SA2 at that point

y0 = [SA1; SA2; ASA1; ASA2];

That hints that inside odefun you need to do

SA1 = y(1); SA2 = y(2); ASA1 = y(3); ASA2 = y(4);