Can't add bending line/Split line cannot be moved after placing/Board region cannot be selected

Solution Details

How to add Bending lines and move Split Lines in a Rigid Flex Design
As these actions affect all layers, make sure, that the Multi-Layer is enabled in the Other Layers section of the Layers area of the View Configuration panel (Press the L key to access this panel from the PCB Editor):




...and the multilayer is active by clicking on it in the respective layer Tab:



For more detailed information about working with Board regions and bending lines please review the following documentation:
https://www.altium.com/documentation/altium-designer/defining-board-regions-and-bending-lines-ad
and
https://www.altium.com/documentation/altium-designer/pcb-obj-splitlinesplit-line-ad