PCB layers shall be capped to "board settings" layers

this is to avoid asking a 8 layer winding on a 4 layer PCB :-D

so make sure that the dropdown PCB layers is maxed out at the correct PCB layer stackup

get it via https://docs.kicad.org/doxygen-python-7.0/classpcbnew_1_1BOARD__DESIGN__SETTINGS.html