As Ron pointed out, the command will take on the other device.
Serial Cables can operate a variable clock rates, or frequencies. This is the duration a voltage is applied to the wire to transmit a single bit. Unlike Ethernet, who's frequencies are built into the specification itself (10BASE-T, 100BASE-TX, etc), Serial Links's frequency can be set differently on each link.
But crucial to transmitting data is for both sides of the cable to use the same Frequency. So a Serial Link elects one side of the cable to be the Clock Master (if you will), and the other side of the cable simply agrees to the frequency set by the Clock Master.
A Serial Cable has a DCE and a DTE side. The DCE side is elected as the Clock Master, so the device which has the DCE side of the cable connected to it must be the one which you enter the
clock rate command.
To determine what side of the cable is connected, you can use the command
show controllers s0/0/0. Four or five lines from the top you'll see it labeled as DTE or DCE.