Chcon: failed to change context of '/usr/local/samba/var/lib/ntp_signd' to ‘system_u:object_r:ntpd_t:s0’: Permission denied

Hello,
According to https://wiki.samba.org/index.php/Time_Synchronisation_-_SELinux_Labeling_and_Policy, I want to set the SELinux, but I got below error:

# chcon -u system_u -t ntpd_t /usr/local/samba/var/lib/ntp_signd
chcon: failed to change context of '/usr/local/samba/var/lib/ntp_signd' to ‘system_u:object_r:ntpd_t:s0’: Permission denied

And:

# ps -eZ | grep ntpd_t
system_u:system_r:ntpd_t:s0        2184 ?        00:00:00 ntpd
#
# sestatus 
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     actual (secure)
Max kernel policy version:      33

Why?

Thanks.