Nonlinear ODE problem

I am using Mathcad Prime Release 9.0 and Datecode9.0.0.0

Trying to use basic ODE solvers like rkfixed within a solve block. My goal is to solve a pair of nonlinear ODEs (intial value problem) which are not linear in the highest derivative term, using state space if possible. I need to use the iterative solver block because reducing the ODEs to standard form is quite difficult. the odesolve function cannot handle these equations. Are the basic solvers usable in a solve block for this type of problem? If so, an example sheet would be very helpful, the documentation and forums have not been enough. I have begun trying to solve the simple spring/mass system this way, before tackling the full equations, but not yet successful.

Here are the errors that I faced
Variable undefined. Unable to form derivative matrix (odesolve).