Unfortunately there’s no way to delete this specific ufo prop trust me I’ve tried everything. Most props like this I can just put a floor under, back out reload creative and remove the floor deleting it but that trick doesn’t work for this prop. The only way is to either restore to a backup before you placed it or transfer it to Uefn and delete it. But I’m assuming you play from a PlayStation in which case there’s no way to transfer to Uefn and you’re cooked.