TopN error when trying to show ten rows in a table

Hi guys, 

Try = TOPN(10,ga1,ga1[Candidate Registrations], DESC)

May be a simple one here but im getting the following error when trying to use the TopN DAX expression to only show the first ten rows of a table: 

The expression refers to multiple columns. Multiple columns cannot be converted to a scalar value.

The syntax highlighter is not showing up any errors, so not sure why this won't work...

Excuse the simplicity as I am new to PowerBi and DAX, any help would be greatly appreciated!

Thanks