I follow your instruction: save putty public and private key, then paste them in to "add ssh key", but no use, there is red notices
This is usually because you're using an older version of PuTTY, which by default created a PuTTY-version of the key instead of the OpenSSH-version.
Newer versions of PuTTY should automatically generate OpenSSH-compatible keys by default.
You can follow this guide to generate a key:
You won't be notified about changes to this idea.