Update: As rechecked on May 20, 2021 this fix is not required anymore. This post is saved for archive purpose only
Since last year (2020), I'm using Raspberry Pi 4 (8GB Version) for my daily work. I was using Raspbian OS, which is quite stable, but I don't like it. So recently I started using Linux Mint (Mate Desktop) 64 Bit edition on Raspberry Pi, and it is quite nice.
As I didn't connected any Audio input to either via HDMI or Aux port, I have to rely on Bluetooth for Sound Output. When doing a fresh install, there is an update for Bluetooth, which cause Bluetooth system unavailable upon the restart.
To fix the problem, I've edited the following file
Following command is added at the very end of the file.
display-setup-script=/usr/bin/systemctl start hciuart
Basically the command run is systemctl start hciuart whenever user log into the desktop. This fix starts the Bluetooth on the machine without any issue and any Bluetooth device can be connected without any problem.