It is possible, that you would like to add something to an already existing Control Board that was created by a closed compound.
Here is an example case:

You have a control board called CAMERAS.

You have this control board because you have a VirtualCam... compound in your scene. The CAMERAS control board is defined inside the VirtualCam... compound.

However the VirtualCam...compound is closed, thus it can't be edited, therefore you can't add any new elements into its CAMERAS control board.

It can be true for other control boards as well. (for example: BILLBOARDS)

You can open these compounds, but we highly advise against it.

Instead, we recommend adding a new control board first and to add the desired compound to the newly created control board.

You can even add elements to this new control board from other control boards.

Like for example the SELECT CAMERA box from the CAMERAS control board.