This error message is caused by the calendar OR user defined calendar not being extended. To resolve this issue you will need to extend the User Defined Calendar. To extend a User Defined Calendar, follow the steps below.
Go to CoreTime.
Select System Params.
Click on Calendars and then select System Calendar.
Select the relevant company, then click User Defined Periods.
Select the Calendar Code.
Select the Calendar Code to extend, then click Generate Period.
Enter Start Date, then select Duration and Number of Periods.
Once the calendar has been extended, you will need to redo your generation of timesheets process again.