If you see the error "Timesheet cannot be modified. Selected time period is already approved." that means that the the timesheets for this period has already been submitted and can't be edited.

To fix this ask your scheduler to Decline the timesheet period to make it editable again

This can be done in the Daily or Weekly Timesheet report (depending on your settings). Click on Decline to unlock the timesheet period for the technician

Learns more about the Timesheets Approval Workflow