sudo apt update
…
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://www.scootersoftware.com bcompare4 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EC34ED227AFAE3F2
W: Failed to fetch https://www.scootersoftware.com/dists/bcompare4/Release.gpg The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EC34ED227AFAE3F2
W: Some index files failed to download. They have been ignored, or old ones used instead.
How to fix this?