System dependencies installation failed

[HarshParmar25]

gnupg is already the newest version (2.2.19-3ubuntu2.1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Hit:1 http://in.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://in.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:3 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:4 https://dl.google.com/linux/chrome/deb stable InRelease
Hit:5 http://in.archive.ubuntu.com/ubuntu focal-backports InRelease
Get:6 https://repo.protonvpn.com/debian stable InRelease [2,519 B]
Err:6 https://repo.protonvpn.com/debian stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 71EB474019940E11
Reading package lists... Done
W: GPG error: https://repo.protonvpn.com/debian stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 71EB474019940E11
E: The repository 'https://repo.protonvpn.com/debian stable InRelease' is not signed.
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.
System dependencies installation failed

[anirudh-hegde]

can

gnupg is already the newest version (2.2.19-3ubuntu2.1). 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. Hit:1 http://in.archive.ubuntu.com/ubuntu focal InRelease Hit:2 http://in.archive.ubuntu.com/ubuntu focal-updates InRelease Hit:3 http://security.ubuntu.com/ubuntu focal-security InRelease Hit:4 https://dl.google.com/linux/chrome/deb stable InRelease Hit:5 http://in.archive.ubuntu.com/ubuntu focal-backports InRelease Get:6 https://repo.protonvpn.com/debian stable InRelease [2,519 B] Err:6 https://repo.protonvpn.com/debian stable InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 71EB474019940E11 Reading package lists... Done W: GPG error: https://repo.protonvpn.com/debian stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 71EB474019940E11 E: The repository 'https://repo.protonvpn.com/debian stable InRelease' is not signed. 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. System dependencies installation failed

I am a beginner in devops, I heard that for this kind problem we can use docker right?