List will not show option to group by

[NearRetirement]

I have a very simple list with about 20 columns and 20 rows. Several of the columns are of type Choice.  I can group by all choice columns but one. Where can I go to see what is blocking me from having the option exposed to group by this column? 

[#2]

never mind. It was set to allow multiple choices which does not allow Group by.