Since a page in Fred4 can be edited by more users at the same time, e.g. different users can work on layers of the same page, certain operations require that you check-out the list of layers first.
For example, if you want to add a layer to a page, or change stacking order, you can do it only if you checked-out the list of layers too.
This is required to protect the integrity of data in a multi-user environment.
Check-out list of page layers:
On the Layers palette, click the List locked icon in the toolbar.
Notes
If the Layers palette is not displayed, to show it, on the Window menu, point to Palettes, and then click Layers.