Hello
effective_io_concurrency can not be set is your system does not support USE_POSIX_FADVISE. In this case only allowed value for effective_io_concurrency is zero.
As far i know mac os does not support posix_fadvise.
Also this is documented behavior https://www.postgresql.org/docs/11/static/runtime-config-resource.html
> Asynchronous I/O depends on an effective posix_fadvise function, which some operating systems lack. If the function is not present then setting this parameter to anything but zero will result in an error.
regards, Sergei