To Add backports packages support for your Debian 11:

1. Edit /etc/apt/sources.list and include following repositories
   

   vim /etc/apt/sources.list

   deb http://deb.debian.org/debian bullseye-backports main contrib non-free
   deb-src http://deb.debian.org/debian bullseye-backports main contrib non-free

2. Update the known repos list to include it

   apt update

   …

3. Install the btop deb package from backports

   apt install -t bullsye-backports btop –yes

   …

Once I installed it, I can straight use it except on some of my Linux machines, which were having a strange encoding $LANG defined, those ones spitted some errors like:

root@freak:~# btop
ERROR: No UTF-8 locale detected!
Use –utf-force argument to force start if you're sure your terminal can handle it.

To work around it simply redefine LANG variable and rerun it

# export LANG=en_US.UTF8

# btop