Systemd service fails to start with status 217/USER and message "Failed to set up user namespacing"

Issue

• systemd service fails to start with the bellow messages:

  Jul 15 16:52:34 rhel-system systemd[17124]: <unit>.service: Failed to set up user namespacing: No space left on device
  Jul 15 16:52:34 rhel-system systemd[17124]: <unit>.service: Failed at step USER spawning /path/to/script: No space left on device
  Jul 15 16:52:34 rhel-system systemd[1]: <unit>.service: Main process exited, code=exited, status=217/USER
  Jul 15 16:52:34 rhel-system systemd[1]: <unit>.service: Failed with result 'exit-code'.
  

• The service definition include options which requires unprivileged user namespaces support to be enabled in the kernel.