GPG error installing pritunl on ubuntu

Hi

After i installed the pritunle using its site instructions, when i want to execute "sudo apt-get update" i get these errors:

GPG error: http://repo.pritunl.com/stable/apt xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7AE645C0CF8E292A

The repository 'http://repo.pritunl.com/stable/apt xenial InRelease' is not signed.

Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

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and after these errors when i start pri tunl and give to it my ovpn file, it connected to my server but didnt open any websites such as facebook and youtube

what should i do?