The error is not coming from your machine, but rather from the remote repository that you've listed. First, to fix that, launch "Software & Updates" from the applications menu and go to the "Other software" tab. There you will find many repositories listed, including the one you added with the commands that you ran.
Select the ones that look anything like this https://swupdate.openvpn.net/community/openvpn3/repos and remove them.
Then, I presume you wanted to install OpenVPN? There are official instructions here:
I noticed that, but the error would've been different. Since the repository was incorrect either way, I thought I'd just omit that part in hopes that using the official instructions would make everything work properly.