This error means your version of ssh-add does not support macOS keychain integration, which allows you to store your passphrase in the keychain.

The -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 -K.

Solving the issue

To add your SSH private key to the ssh-agent, you can specify the path to the Apple version of ssh-add:

/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.

Further reading