[Alneyan]

Only objects within your homesystem (and a single homesystem) can be used in the simulator. So, you can only put the star of your homesystem, the wormholes of your homesystem, and the planets of your homesystem.

You should be able to work out around this problem by using another planet and giving it the cargo of your planet. The feature enabling this is "Change Cargo for player" if memory serves (I am not sure about the actual label)