This article is applicable if you are trying to sort a table after applying the 'group by' filter. 

Sorting (in the ascending or descending order) can be performed on columns shown in Tabular charts and Underlying data. However when 'group by' filters are used, sorting occurs within each group of data and not on the overall list. 

For instance in the example given below, if the data is grouped by 'Priority' and sorting is done on 'Subject', the data within each Priority group will be sorted based on Subject.