Your problem is caused by trying to plot a (2-column) vector Y against a range variable (X). You need to make both expressions in a 2D plot vectors or have the y-expression dependent upon X.
However, in your case, there is no need for X. rkfixed returns a 2-column vector, the first column of which is the list of x-coordinates and the second the y-coordinates (ie, solution to the DE). So, simply use the Mathcad column operator to extract and plot them both.
What version of Mathcad Prime are you using?
Stuart
