Declare all the variables, and for good measure, return the second output from odeToVectorField because it tells you wnat the function is doing internally, and the substitutions it made.
syms c k F0 w y(t)
D1y = diff(y);
D2y = diff(D1y);
eqn = D2y + c*D1y + k*y == F0*sin(w*t) ;
% cond1 = y(0) == 4 ;
% cond2 = diff(y0) == 0 ;
[V,Subs] = odeToVectorField(eqn)
Now, convert this into an anonymous function that the nmumerical ordinary differntial equation solvers can work with, and integrate it to get a numerical result.
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