How to install Avro Phonetic in Ubuntu

sudo apt install ibus-avro

Restart system or Log out - Log in

Go to Settings - Keyboard (Settings - Region & Language in 20.04)

In Input Sources click +

Click Bangla or Bangla (Bangladesh)

Finally select Bangla (Avro Phonetic)

Now it should work just fine.
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If not make sure your Keyboard Input Method System is - IBus
Common Problem
Avro Stops writing Bangla. Try to restart ibus.
ibus restart