Press Alt+Ctrl+T to start terminal and run following command
sudo apt install ibus-avro
Restart system or Log out - Log in
Go to Settings > Region & Language
In Input Sources click +
Click Bangla or Bangla (Bangladesh)
Finally select Bangla (Avro Phonetic)
Now it should work just fine.
If not make sure your Keyboard Input Method System is > IBus
Sometime, Avro Stops writing Bangla. then try to restart ibus.
ibus restart