apt - W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used

I'm facing this error while updating or installing anything in my ubuntu.
Here is the full error:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.mysql.com/apt/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C718D3B5072E1F5
W: Failed to fetch http://repo.mysql.com/apt/ubuntu/dists/bionic/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C718D3B5072E1F5
W: Some index files failed to download. They have been ignored, or old ones used instead.

How can I solve all this type of errors?