Can't add or replace planets/stars and others in creating system

i was making my own system, ive finished at least a star and a planet and made a launch pad on it, but when i was creating a system, i couldn't replace the star that comes in the system with the star that i made and i couldn't also add planets to it, and also when im trying to save it when i add any planet (from the stock, community or from the planet studio) it shows that the system contains no matching element.