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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:
Defining Board Regions & Bending Lines
and
Split Line