Using Manage Aggregations - Count table rows always gives error

Hi,

I am having an issues with using the Manage Aggregations function.  In any model I build I get the same error when I try to 'Count table rows'

I select the summarization 'Count table rows'

I select the Details Table 'fact table'

Now as far as im aware you cant/dont select the 'Details column' and this makes sense

When I click on Apply all, I get this error everytime:

Is this a bug or am I missing something here.