I had the same issue after a power outage. No voice assist selected, and a new one ( Bixby or Alexa) can not be selected, the selection menu simply would not pop up. I tried the recommended remedies of re-charging the remote, re-pairing it, soft-resetting the tv, no luck.
I ended up resetting the tv to factory defaults, and during the re-setup, it became apparent that my Samsung account was locked out due to too many incorrect login attempts. After clearing the account lockout (had to reset the password) , I was able to select voice assist . I am not totally sure if it just needed a reset, or if it was the locked out status of the account. I suspect the account lockout, because other people reported the same symptom when the user agreements are not checked of as "agreed".
So if this happens the first thing to try is loggin into the Samsung account via browser, and making sure the login is working. This either fixes the issue, or worst case, a subsequent factory reset of the TV will.
