Python Add-In ESRI Digital Sign Utility

In using the ESRISignAddIn.exe utility, I have encountered an error. Add-In is functional. I copied the ESRISignAddin.exe over to my ArcGIS\Desktop10.2\bin folder. When I run the utility, I choose my addin, then I choose my cetificate. After I hit "Finish" I get this error that says "Signing key is not loaded" (see screenshot), followed by a message that says "Failed to sign the add-in". The certificate manager indicates that the certificate is "ok".

Any ideas on what this error means, and how to fix?


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