You need to create weekly pay periods from Employee > Settings > Pay Period > Custom so that the same pay periods appear as drop-downs in the Employee Payroll Summary report. Once you see your updated pay periods from the Payroll Summary report, select the appropriate pay period and click the Calculate Commissions Now button. If you are sure that you will continue to pay your employees on a weekly basis, go ahead and create your pay periods for up to 3 months or more, to avoid revisiting this section each time. 

At the same time, do ensure that the list of pay periods is not too long. For example, if you want to configure weekly pay pay periods for 3 months, it means there will be about 12-13 pay periods (or 12-13 options in the drop-down list in the Employee Payroll Summary report). 

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