Welcome to programming! All JavaScript programmers hate the JavaScript console, so you will fit right in 
Even though we hate the JavaScript console, we learn to use it as well as we can because it can be amazingly helpful – as you found out.
When I enter your code in, I see the following in the JavaScript console:
VM151 code.html:7 Uncaught TypeError: Cannot set properties of undefined (setting 'ammo')
at VM151 code.html:6:21
at code.html:10:12
Line #6 (and #7) are:
Physijs.script.ammo = '/ammo.js';
Physijs.script.worker = '/physijs_worker.js';
The problem here is that we need to set the ammo and worker properties on Physijs.scripts (ending with an “s”), but the code currently has Physijs.script (no “s” at the end).
When I change those two lines to:
Physijs.scripts.ammo = '/ammo.js';
Physijs.scripts.worker = '/physijs_worker.js';
Then I see the sky and the ground. Hopefully that fixes it for you as well.
-Chris
P.S. It is easier to post code in here if you follow the instructions in this post.