Update - A fix was released for this issue on 1/11/2021: An error was occurring when adding an attendance leave record in schools using the Calculate Attendance Totals Based on Period Attendance option, fixed.
For schools that have the School Settings option Calculate Attendance Totals Based on Period Attendance enabled, this produces a Value cannot be null error while dropping a student from attendance using the Update Attendance button on the Demographics page or when adding a Leave record on the Attendance Enrollment page.
This option, on occasion, can also cause the Monthly Attendance Summary reports to fail.
A workaround is to uncheck the Calculate Attendance Totals Based on Period Attendance option on the School Settings page to allow a student to be dropped from attendance or to run the Monthly Summary reports.
A fix is currently being worked on and we will update this post when an update is released.
The following Aeries Idea has also been created to keep users updated. https://ideas.aeries.com/forums/926386/suggestions/42278215