You omitted the equation d/dt Z(t) = VZ(t) and instead provided the initial condition VY(0)=0 twice
the main error: You type d/dt as a fraction and ended up with d/dt*X(t), a multiplication. You have to use the derivative operator from the "calculus" palette or use the appropriate keyboard shortcut. In a solve block with ode you may also use the prime operator to apply the derivative. You get it by pressing Ctrl-F7 and you can see it in Francescos sheet. The derivative operator takes up more space but provides better visibility.
In one plot you used X instead of x (you assigned the result of the solve block to variables in lower case).
Remaining valid the observations made by Werner and his sheet, I suggest you that you can also use the units of measurement for each constant and variable, with the foresight, however, to make them dimensionless before the solution block, as shown in the photo:
