You have a fixed dimension, a driven one and another one which is calculated based on the driven dimension.
You lack a driving dimension.
and it's where your calculation fails.
It's called Potential Circular reference.
All dimensions are locked or driven.
You need a driving dimension to decide where your circle is placed, then the other dimensions can be calculated based on the driving one.
Right click d1 dimension and make it driving.
then you can double click it and change it to be half of the fixed dimension.