Avro keyboard not working

I have installed avro kb but sometimes it not working properly. When logout it worked properly for sometimes. and then english again without any notice. no bangla. how to solve this issue?

What Regional settings and Keyboard language settings have you chosen?

I am sorry, I don't know where should I change.

The language is set to English, for the system. You will have to change that, then your kb settings should remain in that language.

how to change language?

Looking at your screenshot, you have English above Bangla as input source choices.
I am not sure if you can promote Bangla to first input source option usong the settings cog icon for input sources.
If you want Bangla keyboard input only, put it to top of list and/or delete English input option from that list.

I have changed but the same issue, change Bangla to English after a few seconds. actually, I am trying to write in Bangla.

It may work better by the terminal, how to change your locale using this link. It essentially has you use the sudo dpkg-reconfigure command to set your language and pick the primary. Then you log out and back in to see the changes (which should also fix your keyboard issue).