why cant i sign in to my apple tv?

what do you mean ?

first the appletv sign into the wifi network if one does not use ethernet

second it can connect to the shared lib of computers on the network with iTunes running and shared lib enabled

third it can connect to iTunes server for renting and the likes

forth it can connect to other third party apps if it's 4g or 4k model

then iOS devices and computers can airplay to it

what is that which give you problems ?