lavamaster@lavamaster:~$ wget --no-check-certificate https://apt.lavasoftware.org/lavasoftware.key.asc
--2020-11-27 17:45:41--  https://apt.lavasoftware.org/lavasoftware.key.asc
Resolving apt.lavasoftware.org (apt.lavasoftware.org)... 51.148.40.41
Connecting to apt.lavasoftware.org (apt.lavasoftware.org)|51.148.40.41|:443... connected.
WARNING: The certificate of ‘apt.lavasoftware.org’ is not trusted.
WARNING: The certificate of ‘apt.lavasoftware.org’ doesn't have a known issuer.
HTTP request sent, awaiting response... 200 OK
Length: 5412 (5.3K) [text/plain]
Saving to: ‘lavasoftware.key.asc’

lavasoftware.key.asc                               100%[================================================================================================================>]   5.29K  --.-KB/s    in 0s      

2020-11-27 17:45:42 (34.7 MB/s) - ‘lavasoftware.key.asc’ saved [5412/5412]

lavamaster@lavamaster:~$

lavamaster@lavamaster:~$ sudo apt-key add lavasoftware.key.asc
OK
lavamaster@lavamaster:~$

lavamaster@lavamaster:~$
lavamaster@lavamaster:~$ cat /etc/apt/sources.list
# See https://wiki.debian.org/SourcesList for more information.
deb http://deb.debian.org/debian buster main contrib non-free
deb-src http://deb.debian.org/debian buster main contrib non-free

deb http://deb.debian.org/debian buster-updates main contrib non-free
deb-src http://deb.debian.org/debian buster-updates main contrib non-free

deb http://security.debian.org/debian-security/ buster/updates main contrib non-free
deb-src http://security.debian.org/debian-security/ buster/updates main contrib non-free

deb  https://apt.lavasoftware.org/release buster main

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

deb http://ftp.debian.org/debian stretch-backports main contrib non-free

deb-src http://ftp.debian.org/debian stretch-backports main contrib non-free
lavamaster@lavamaster:~$

lavamaster@lavamaster:~$ sudo apt update
Hit:1 http://deb.debian.org/debian buster InRelease
Hit:2 http://deb.debian.org/debian buster-updates InRelease                                      
Hit:3 http://deb.debian.org/debian buster-backports InRelease                                                          
Hit:4 http://security.debian.org/debian-security buster/updates InRelease                                              
Hit:5 http://ftp.debian.org/debian stretch-backports InRelease                                   
Ign:6 https://apt.lavasoftware.org/release buster InRelease                
Err:7 https://apt.lavasoftware.org/release buster Release
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 51.148.40.41 443]
Reading package lists... Done
E: The repository 'https://apt.lavasoftware.org/release buster Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
lavamaster@lavamaster:~$