Drag Failure Mode to drop zone for row (as a grouping column).
Drop Date on top of N (again, use Date as a grouping column ).
Drag % of Total under (or over, depends on which you want to come first) the label Date.
Drag "Date" from the column heading and drop it onto "% of Total".
Alternatively, you can drag Date from your column list and drop it onto "% of Total".
The idea is to drop the name "Date" to "% of Total" to let tabulate know that you want to use its total for the denominator.
The following script will generate the table that I think you want -
Tabulate(
Add Table(
Column Table(
Grouping Columns( :Date ),
Statistics( Name( "% of Total" )(Date) )
),
Row Table( Grouping Columns( :Failure Mode ) )
)
);