@minkimoo,
Make sure that you have navigated to the correct client list by comparing to the images here: Blacklisting Devices Through the Tether App
If the option still does not appear, the feature can easily be configured from the web interface of Archer Routers
It may be, as is the case with some other routers that I have used, that the device that you are attempting to block needs to be connected to the network at the time. Are you able to block devices that you know have an active connection to the network? If not, I would recommend reinstalling the app or attempting to block the devices from another device using the tether app.