Newbie question: git tag failure

From: Aubrey <aubreylee@gmail.com>
Date: Mar 2, 2006, 15:51 UTC
Message-ID: <6d6a94c50603020751v4bb96b75q1686d36b32e2c40f@mail.gmail.com>

There are two commit. I want to tag one commit by commit id, but failed. Thanks for any hints.

Regards, -Aubrey

aubrey@linux:~/local/mygit> git log
===============================================================
commit a8ac6540cf96f79225cc8457b02cb90d40ac7991
Author: Aubrey.Li <aubrey@linux.site>
Date:   Thu Mar 2 23:34:18 2006 +0800

    add commit log

commit 43368b967bd838292b8cc6b576c5aa13850551cc
Author: Aubrey.Li <aubrey@linux.site>
Date:   Thu Mar 2 22:24:55 2006 +0800

    Initial commit
===============================================================
aubrey@linux:~/local/mygit>git tag -u
43368b967bd838292b8cc6b576c5aa13850551cc v2.5
gpg: directory `/home/aubrey/.gnupg' created
gpg: new configuration file `/home/aubrey/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/aubrey/.gnupg/gpg.conf' are not yet
active during this run
gpg: keyring `/home/aubrey/.gnupg/secring.gpg' created
gpg: keyring `/home/aubrey/.gnupg/pubring.gpg' created
gpg: skipped "43368b967bd838292b8cc6b576c5aa13850551cc": secret key
not available
gpg: signing failed: secret key not available
failed to sign the tag with GPG.