Win10-WSL: problems installing Nektar on Ubuntu 18.04

[duwujianghu]

I want to install Nektar++ on Win10-WSL: Ubuntu 18.04 using the pre-compiled
packages, but I encountered a problem.

After
Code:
sudo apt-get update

I get the following:

Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Hit:3 http://archive.ubuntu.com/ubuntu bionic InRelease
Get:4 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:1 https://www.nektar.info/ubuntu-bionic bionic InRelease [6114 B]
Get:5 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Err:1 https://www.nektar.info/ubuntu-bionic bionic InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DB5E71FF7AC99B37
Reading package lists... Done
W: GPG error: https://www.nektar.info/ubuntu-bionic bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DB5E71FF7AC99B37
E: The repository 'http://www.nektar.info/ubuntu-bionic bionic 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.

I have tried the following:
Using different servers such as the following:

code:
curl -sL http://keyserver.ubuntu.com/pks/lookup?op=get&search=0xDB5E71FF7AC99B37 | sudo apt-key add
gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv DB5E71FF7AC99B37
curl -sL "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0xDB5E71FF7AC99B37" | sudo apt-key add
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys DB5E71FF7AC99B37
sudo apt-key adv --hkp://keys.gnupg.net--recv-keys DB5E71FF7AC99B37
gpg --keyserver hkp://pgpkeys.mit.edu --recv DB5E71FF7AC99B37

But, no good results.The key could not be found.Examples are listed below:

Executing: /tmp/apt-key-gpghome.IoDko6URV5/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys DB5E71FF7AC99B37
gpg: keyserver receive failed: No data

gpg: no valid OpenPGP data found.

Have tried to see if key exists with command:
code:

apt-key list

but it is not listed there.

/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-archive.gpg

pub rsa4096 2012-05-11 [SC]
790B C727 7767 219C 42C8 6F93 3B4F E6AC C0B2 1F32
uid [ unknown] Ubuntu Archive Automatic Signing Key (2012) [email protected]

/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-cdimage.gpg

pub rsa4096 2012-05-11 [SC]
8439 38DF 228D 22F7 B374 2BC0 D94A A3F0 EFE2 1092
uid [ unknown] Ubuntu CD Image Automatic Signing Key (2012) [email protected]

/etc/apt/trusted.gpg.d/ubuntu-keyring-2018-archive.gpg

pub rsa4096 2018-09-17 [SC]
F6EC B376 2474 EDA9 D21B 7022 8719 20D1 991B C93C
uid [ unknown] Ubuntu Archive Automatic Signing Key (2018) [email protected]

As you can see, many avenues explored with no good results.
Looking for some help, please