As for not being able to place things, that occasionally happens to me because (1) plants sometimes drop their harvest on the ground or (2) harvestables given to toddlers are consumed but then end up on the floor anyway.
If it's outdoors, I place the white color of free concrete flooring (it's called something like Construction-Quality Concrete) on the ground, which allows me to see any fallen harvestables.
But wait, your game tells you there's nothing to delete? Weird...