Thanks and I just tried it with following result:
rock@Debian-Desktop:~$ dpkg --add-architecture arm64
dpkg: error: unable to create new file '/var/lib/dpkg/arch-new': Permission denied
rock@Debian-Desktop:~$ sudo dpkg --add-architecture arm64
[sudo] password for rock:
rock@Debian-Desktop:~$ apt update
Reading package lists... Done
W: chmod 0700 of directory /var/lib/apt/lists/partial failed - SetupAPTPartialDirectory (1: Operation not permitted)
E: Could not open lock file /var/lib/apt/lists/lock - open (13: Permission denied)
E: Unable to lock directory /var/lib/apt/lists/
W: Problem unlinking the file /var/cache/apt/pkgcache.bin - RemoveCaches (13: Permission denied)
W: Problem unlinking the file /var/cache/apt/srcpkgcache.bin - RemoveCaches (13: Permission denied)
rock@Debian-Desktop:~$ sudo apt update
Hit:1
http://security.debian.org stretch/updates InRelease
Ign:2
http://ppa.launchpad.net/utappia/stable/ubuntu focal InRelease
Err:5
http://ppa.launchpad.net/utappia/stable/ubuntu focal Release
404 Not Found [IP: 91.189.95.83 80]
Get:6
http://security.debian.org stretch/updates/main arm64 Packages [492 kB]
Ign:3
http://cdn-fastly.deb.debian.org/debian stretch InRelease
Get:4
http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease [91.0 kB]
Get:8
http://security.debian.org stretch/updates/contrib arm64 Packages [1760 B]
Get:9
http://security.debian.org stretch/updates/non-free arm64 Packages [1272 B]
Hit:7
http://cdn-fastly.deb.debian.org/debian stretch Release
Get:10
http://cdn-fastly.deb.debian.org/debian stretch-updates/main arm64 Packages [27.9 kB]
Get:12
http://cdn-fastly.deb.debian.org/debian stretch/main arm64 Packages [6927 kB]
Get:13
http://cdn-fastly.deb.debian.org/debian stretch/contrib arm64 Packages [39.9 kB]
Get:14
http://cdn-fastly.deb.debian.org/debian stretch/non-free arm64 Packages [50.8 kB]
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/utappia/stable/ubuntu focal 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.
rock@Debian-Desktop:~$
Any thoughts? Thanks! :-)