How can I assign weekly off to staff?

You can assign WO on the staff category level or site level from

Dashboard -> Workforce plan -> Weekly Off -> Add

or

Via bulk spreadsheet available for shift and WO plan

Dashboard -> Workforce Plan -> Shifts&WO -> Schedule shifts and WO (download Excel sheet) -> Fill WO in front of the dates you want to assign WO to staff -> Reupload the sheet back

Admin/Manager can also assign from user app – User App > Time Tracking > Shifts and Weekly Off > Plan WO