According to the this commit (May 1,2020) which is commited to the unstable
version, your syntax is correct. But it is not released to the stable versions such as the one you used.
If you want to remove client by ip:port format then you need something like this;
127.0.0.1:6379> client list
id=272 addr=127.0.0.1:51374 fd=8 name= age=66 idle=1 flags=N db=0 sub=0 psub=0 multi=-1 qbuf=26 qbuf-free=32742 obl=0 oll=0 omem=0 events=r cmd=client
id=273 addr=127.0.0.1:51376 fd=9 name= age=19 idle=16 flags=P db=0 sub=1 psub=0 multi=-1 qbuf=0 qbuf-free=0 obl=0 oll=0 omem=0 events=r cmd=subscribe
127.0.0.1:6379> client kill 127.0.0.1:51376
OK
127.0.0.1:6379>