You'll get that error if there are blank cells, or text entries, in the
date column in the source table. There are suggestions here for fixing
the problem:

http://www.contextures.com/xlPivot07.html#Problems

Tufail wrote:
Hello Sir,

I have some problam in Pivot Table Date grouping, actully i want make
grouping Year & Month, what i am donig for this setting as follow.
1. selecting date in pivot tabe -- group and Show Detail -- Group , and
there this error is coming out -- can not group that selection.
Now what I should please let me know, thanks in advance.


--
Debra Dalgleish
Contextures
http://www.contextures.com/tiptech.html