Signature verification fails for program that I thought I removed. How to remove the program completely?

Here is the output that I get

    53 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://deb.opera.com/opera-stable stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4B8EC3BAABDC4346
W: Failed to fetch https://deb.opera.com/opera-stable/dists/stable/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4B8EC3BAABDC4346
W: Some index files failed to download. They have been ignored, or old ones used instead.

But, I thought that I had removed Opera. I guess I didn't do all of the needed steps. How can I get this error to stop recurring given that I don't want the underlying program?

You added a repository for Opera. To remove it, open Software and Updates, click on the Other Software tab, and untick the box next to the Opera repo.