The Y-axis type depends on what type of aggregation you are performing.
Since the Y-axis is depending on a number to plot, it will not allow
you to select text fields like 'name' unless you have turned on
grouping... and then selecting something like count. A count of names
will be a valid Y-axis field... In your case, since you are counting...
you could just put the number '1' in the Y-axis and get the same results
as if you put a field in there.