(48G 49G 50G) Runge-Kutta Diff Eqn solver

[Werner]

You're in exact mode, and so [0 0] is NOT the same as [0. 0.].
The former is a symbolic matrix, the latter a numeric, and that is what we need here (and why you're getting the 'Bad Argument Type')
Provide values with decimal points throughout and it will work.
(or, switch on approx. mode: MODE/CAS and select Approx).

Cheers, Werner