The digital ID that you are trying to use to create the digital signature is supposed to be in the Windows Certificate Store, but it's missing, or at least the private key portion of the digital ID is missing. Is your digital ID on a token or smart card? If it is then Windows doesn't really have access to the private key, it just points back to the hardware device even though it looks like the digital ID (also known as a signing credential) is install in the Windows Certificate Store. If you are using a hardware device make sure it's plugged in. If you're not using a hardware device you can try to import the digital ID from a file back into Windows, but you have to have access to the P12/PFX file that contains the digital ID.