I have for some time not been able to use the build tools like placing a cube entity in world… @Judas suggested I might be “out of bounds” and that the entity was trying to be put below Y=0 and failing.
It turns out that if my Ron avatar was up in the air a bit the cube entity placement icon/tool did work. hence the problem is that an avatar (Ron) who is set to collide with world boundaries and object environmental gravity settles at Y=0, but as noted elsewhere he is ACTALLLY centred at Y=0 and his feet are below that. The cube entity fails if its used in this normal gravity position.
This all needs fixing. The cube should be placed anyway and just adjusted to be at or above Y=0. And as noted elsewhere, all avatars need to have their Y capsule adjusted as suitable so hey settle on the actual Y=0 for the bottom of their feet and physics collision shape.