SSRS - How to use Rownumber

=Rownumber("Purchase_Header_No_1")

1. You must use the EXACT name of the group you are counting in enclosed in " "
2. OR you must use the EXACT name of the Table in which you are using rownumber
3. OR you must use the EXACT name of the report dataset

Examples: Show or hide based on rownumber

=iif(Rownumber("Table1_Group1")>12, True, False)