On our debian buster build system which we still use there was a compile error that Kleo::Settings does not have a member settings.isValidityPeriodInDaysImmutable
I think the problem might be that the kconfig_compiler is in a too old version. If this is easy to change in an older syntax please do so.
I want to update our build system to debian stable (bullseye) anyway where I did not have this problem.