Getting Error "cannot group that selection pivot table"

Hi All,

I am trying to group the pivot table based on special character available on the string but getting the error "cannot group that selection pivot table".
I changed all blank cells in the table as NA, so my PIVOT doesn't have any blank cells.

Sample:

A
IT001
IT001-01
IT001-02
IT002
IT002-01
IT003
IT004
IT004-01