When all of the values for an Analysis Field are the same, the tool cannot use that field in the grouping algorithm. The field will still appear in the output feature class.
This is a warning and may actually be reflecting expected behavior. Workflows that iteratively decrease the number of features analyzed, for example, could lead to a set of features that have all the same values for Analysis Fields. If you weren't expecting this message, check the Analysis Fields you selected to make sure they have a variety of values that the Grouping Analysis tool can use to partition your features into distinct groups.