There are a couple of reasons the audience size estimator could be unavailable.
If you have a segment which is created with an 'in session' clause, Permutive will be unable to calculate the audience size. However, once you have set up the segment, you can see its growth in the analytics reports.
Additionally, at times, the estimator will need to be re-calculated by manually pressing the 'Calculate' button at the bottom of your segment builder.
If these suggestions do not resolve the issue you are having with the audience size calculator, please get in touch by contacting email@example.com or using the red live chat icon in the bottom right corner of your Permutive dashboard.