I get this message when trying to use the mail app, which I do infrequently. I have no idea of what the login keychain password is, have no recollection of ever having created one, don't know what "privateKey" is, get no results in an attempt to search for it, can't use the mail app without it and cannot get this message to disappear without restarting.
The password to the login keychain is usually the login password for your macOS login.
Why are you getting that password prompt? There's a certificate in use with your Mail, probably for encryption, and you can remove that to clear that prompt. That means using Keychain Access, and selecting My Certificates from the list of choices within the app. You'll then need to find which certificate is being used with your Mail, export a copy, and then delete it. Or enter your login password, and allow Mail to use the certificate.
This is the mail message signing certificate that is triggering that prompt:
Sign or encrypt emails in Mail on Mac - Apple Support
The password to the login keychain is usually the login password for your macOS login.
Why are you getting that password prompt? There's a certificate in use with your Mail, probably for encryption, and you can remove that to clear that prompt. That means using Keychain Access, and selecting My Certificates from the list of choices within the app. You'll then need to find which certificate is being used with your Mail, export a copy, and then delete it. Or enter your login password, and allow Mail to use the certificate.
This is the mail message signing certificate that is triggering that prompt:
Sign or encrypt emails in Mail on Mac - Apple Support
I get this message in the mail app and don't know what it means
