systemd unit file:
[Service]
SystemCallFilter=[...]
Error message:
unit-name.service: Main process exited, code=killed, status=31/SYS
How to find out which system call is missing in the whitelist?
You can first get the system call that failed with
journalctl _AUDIT_TYPE_NAME=SECCOMP
Syscall codes (such as syscall=28) are listed in a header under /usr/include. For example, on Debian x86_64, it's in /usr/include/x86_64-linux-gnu/asm/unistd_64.h.