OS X productsign error: Could not find appropriate signing identity

I had the same issue. My installer certificate had no private key. You can check this by opening keychain and look for your certificat named like:

Developer ID Installer: My Company (1258345791ZL)

If you there is a private key attached, you should see

Developer ID Installer: My Company (1258345791ZL) (!) Private Key

If you cannot unfold the private key under your certificate, it means that it is missing.

Then to fix this, you have to log on to developer.apple.com with your apple ID and

1- try remove and download your certificate again

2- if this is not enough create a new with with the following steps

2a - click on the (+) or go here:

https://developer.apple.com/account/mac/certificate/distribution/create

2b - specify Production / Developer ID, then press Continue

2c - specify "Developer ID Installer"

2d - follow, the instruction to create a certificate request using keychain

2e - upload your certificate request file in apple developper

2f - download the newly created certificate in your keychain.

2g - cleanup all other installer certificate to be sure.

Et voila, you can sign.

Check for the private key in your keychain. You need both the key and the certificate to do the signing (together Apple calls this an "identity").