The pop-up is not available for the Memo column in the Weekly Timesheet using the Employee Center Role in the NetSuite 2023.1 version.
This is a standard behavior in the Weekly Timesheet using an Employee Center role. No pop-up is available in the Employee Center when the Memo field is displayed as a column. Nevertheless, the user can still expand the Memo column by adjusting the column length so that the whole field is visible without using the pop-up.