There is are a set of changes to HASS OS that may fix this for most people
If Home Assistant is complaining about missing firmware, in a terminal do the following:
# Based on: https://gist.github.com/rometsch/dfd24fb09c85c1ad2f25223dc1481aaa
# Make firmware directory
mkdir /lib/firmware/rtl_bt/
# install 7zip
apk add p7zip
# Download and extract mpow_BH519A drivers (contains Realtek BT firmware)
wget https://mpow.s3-us-west-1.amazonaws.com/mpow_BH519A_driver+for+Linux.7z
7zip -d mpow_BH519A_driver+for+Linux.7z
# For Realtek RTL8761B chip
mv 20201202_LINUX_BT_DRIVER/rtkbt-firmware/lib/firmware/rtl8761bu_fw /lib/firmware/rtl_bt/rtl8761b_fw.bin
mv 20201202_LINUX_BT_DRIVER/rtkbt-firmware/lib/firmware/rtl8761bu_config /lib/firmware/rtl_bt/rtl8761b_config.bin
# clean up
rm -rf 20201202_LINUX_BT_DRIVER
Reboot the device/VM