What is the difference between these two sheets - one works one doesn't.

In the 'does not work' sheet, you have added pi as a unit to the result. It's not defined as such. Pi is a constant by default; you can (also) define it as a unit. But you have to do that yourself.

In the 'works' sheet you have added pi as an automatic variable  (check label, it says  '-' which means 'automatic' ). That's where Prime may find out for itself, and that apparently works.

Success!
Luc