The ATD service is installed along with the at command. This service is required to be running for the at command to work
Install at by using your package manager. For example, if you are using yum as a package manager, type in:
# yum install at
To start the ATD services on newer systems, type in:
# systemctl start atd
For older systems type in:
# /etc/init.d/atd start
Follow your system admin procedures to make sure the ATD service starts up automatically on system boot.