git commit signing failed: secret key not available

What worked for me was adding

git config --global gpg.program "C:/Program Files (x86)/GNU/GnuPG/gpg2.exe"

If you want to find the full path of gpg2.exe:

where gpg2.exe

This worked for me on Windows 10 (Note that I use the absolute path to gpg.exe):

git config --global gpg.program "C:\Program Files (x86)\GnuPG\bin\gpg.exe"

This was the error I got prior to the fix:

gpg: skipped "3E81C*******": secret key not available
gpg: signing failed: secret key not available
error: gpg failed to sign the data
fatal: failed to write commit object

You need to configure the secret key before using it.

git config user.signingkey 35F5FFB2

Or declare it globally if you want to use the same key for every repository.

git config --global user.signingkey 35F5FFB2

Source: Git Tools - Signing Your Work