Likely, your shell is not `bash` but `dash` which doesn't have `disown`. Do `fgrep $(id -n) /etc/password` and see whether your shell is `bash`. If it's not, then update it, re-login, and you will be able to disown processes.
If you want to continue using the current shell (which lacks the `disown` builtin) you can use `nohup your_command_here &` to achieve almost the same behaviour.
The difference between `disown` and `nohup` is that `disown` prevents shell from sending HUP to disowned process, while `nohup` installs a handler that ignores the HUP signal sent by shell at the exit.