In Financial Plans > Cost Plan (New UX), cannot enter Planned Cost amounts in the Planned View or Planned to Actuals View.
If the line is an unplanned line the line cannot be edited.
If this is a planned line or a new line it may be caused by double clicking on the cell.
Release : 15.9.1
Click only once on the cell. If you double click and the cell turns white you may not be able to edit the cell.