This error means your version of
ssh-add does not support macOS keychain integration, which allows you to store your passphrase in the keychain.
-K option is in Apple's standard version of
ssh-add, which stores the passphrase in your keychain for you when you add an ssh key to the ssh-agent. If you have installed a different version of
ssh-add, it may lack support for
Solving the issue
To add your SSH private key to the ssh-agent, you can specify the path to the Apple version of
/usr/bin/ssh-add -K ~/.ssh/id_rsa
Note: If you created your key with a different name, or if you are adding an existing key that has a different name, replace id_rsa in the command with the name of your private key file.