The package uses sys/io.h on an architecture that doesn't support it. The sys/io.h header is architecture-dependent and therefore rxtx needs to be ported to AArch64 or placed on the blacklist. Particularly src/portmode.c and src/RawImp.c unconditionally include sys/io.h without checking if the non-portable header is present and disabling the functionality that requires ioperm et. al.