А почему?
Смотри systemctl status MyTimer.timer и journalctl -u MyTimer.timer
May 13 19:51:46 localhost.localdomain systemd[10263]: backup.service: Failed to determine supplementary groups: Operation not permitted
May 13 19:51:46 localhost.localdomain systemd[10263]: backup.service: Failed at step GROUP spawning /usr/bin/nautilus: Operation not permitted
May 13 19:51:46 localhost.localdomain systemd[1458]: backup.service: Main process exited, code=exited, status=216/GROUP
May 13 19:51:46 localhost.localdomain systemd[1458]: backup.service: Failed with result 'exit-code'.
May 13 19:52:16 localhost.localdomain systemd[1458]: backup.service: Service RestartSec=30s expired, scheduling restart.
May 13 19:52:16 localhost.localdomain systemd[1458]: backup.service: Scheduled restart job, restart counter is at 23.
May 13 19:52:16 localhost.localdomain systemd[1458]: Stopped Backup my files.
May 13 19:52:16 localhost.localdomain systemd[1458]: Started Backup my files.
May 13 19:52:16 localhost.localdomain systemd[10264]: backup.service: Failed to determine supplementary groups: Operation not permitted
May 13 19:52:16 localhost.localdomain systemd[10264]: backup.service: Failed at step GROUP spawning /usr/bin/nautilus: Operation not permitted
May 13 19:52:16 localhost.localdomain systemd[1458]: backup.service: Main process exited, code=exited, status=216/GROUP
May 13 19:52:16 localhost.localdomain systemd[1458]: backup.service: Failed with result 'exit-code'.
May 13 19:52:46 localhost.localdomain systemd[1458]: backup.service: Service RestartSec=30s expired, scheduling restart.
May 13 19:52:46 localhost.localdomain systemd[1458]: backup.service: Scheduled restart job, restart counter is at 24.
May 13 19:52:46 localhost.localdomain systemd[1458]: Stopped Backup my files.
May 13 19:52:46 localhost.localdomain systemd[1458]: Started Backup my files.
May 13 19:52:46 localhost.localdomain systemd[10272]: backup.service: Failed to determine supplementary groups: Operation not permitted
May 13 19:52:46 localhost.localdomain systemd[10272]: backup.service: Failed at step GROUP spawning /usr/bin/nautilus: Operation not permitted
May 13 19:52:46 localhost.localdomain systemd[1458]: backup.service: Main process exited, code=exited, status=216/GROUP
May 13 19:52:46 localhost.localdomain systemd[1458]: backup.service: Failed with result 'exit-code'.