Monthly column reporting is not available on Sales by Partner Summary report.
As an alternate solution the user may customize the Sales by Sales Rep Summary to show Sales by month for Primary partner.
- Navigate to Reports > Sales > Sales by Partner > Customize Summary.
- Remove the Sales Rep field under the Report builder page by clicking on the customize button from the report.
- Under Add fields > Sales > Primary partner > choose name and make it the first column of the report
- Under the sorting page, remove the Sales rep field and replace it with Primary partner > name column.
- On the Report footer, choose Month under Column.
- Under Date column choose custom and assign the time period you want the report to run by in the From and To columns