Hi - when I started Jami (on Ubuntu Focal) and set up my account, I could only do it by unselecting “register username” (when I selected it then “next” was greyed out).
After setting the account up that way I was surged again to register, as the account only exists on my device. In the “account” preferences when I type a username, the text box becomes red, and gives a pop-up message “lookup unavailable, check your network connection”.
As far as I know there are no network problems. Could it be that I’ve missed a setting / box to tick?