If when validating your Placement, the message advises for the Placement to be Finalised and saved, and the slider/field is locked, click Validate again and it will then unlock and then the action can be run.
The Finalised slider is usually on the Placement Details tab:
If you do not have this option, use the dropdown on the top right of the record:
The reason that the Finalise field can sometimes be locked, is because it only needs to see the Placement Details tab and the Addresses tab in order to Finalise. If all the tabs are visible, then Finalised will be locked.
How it should look:
How it looks when normal/locked: