OP Generation Failed

Hi! I've also had this issue before when placing lots from the gallery and the way i fixed the issue was changing any and all platforms to ground level, effectively deleting them. For some reason it was interfering with the build; nothing else worked for me either, not going into manage worlds, not saving, not deleting any items, not even quiting the game entirely, super frustrating but thankfully easily solvable! Hope this helps for anyone else having this issue.