It looks as if you don't need to solve it numerically.
syms x(t)
xpp = diff(x,t,2)
syms k
dsolve(xpp + k*sin(x) == 0)
dsolve(xpp + k*sin(x) == 0)
ans =
0
2*jacobiAM((2^(1/2)*(C1 - k)^(1/2)*(C2 - t)*1i)/2, -(2*k)/(C1 - k))
-2*jacobiAM((2^(1/2)*(C1 - k)^(1/2)*(C2 - t)*1i)/2, -(2*k)/(C1 - k))
Of course, it would help if you had some initial or boundary conditions. Then you might get a better answer.
But if you really, really need to solve it numerically, then you need to start with a tool like ODE45, and you need to pose a set of initial conditions, etc. As well, you need to define the value of k. No numerical solution can be found unless you specify k as a NUMBER.
