The following option is grayed out in the Modern User Experience (UX), not selectable: Add Assigned Tasks. The button to assign tasks is not working
The options are grayed out because it does not apply to the user/timesheet for the given time.
This option is enabled only when you have assignments. This option populates a timesheet with task assignments that are currently open.
The open assignments have a date range within the time period of the new timesheet and typically have non-zero pending or accepted ETC. The range also includes the number of days that your administrator set on the Timesheet Options page. The Populate Time Range field includes a setting for the days before and after the time period start (default = 7 days). If any open assignments have non-zero ETC, the tasks in any future time periods outside the range also appear.