If you list the certificate that gives you this error, you will see : Private Key = NONE.
The main reason is that you sent a certificate for signature and then added the signed certificate with the wrong label.
For instance, you sent a certificate with label ibmWebSphereMQCSQ2 and added the signed certificate with label ibmWebSphereMQCSQ1. The latter will not have a private key.
Add the signed certificate with the same label of the original, self-signed certificate sent for signature.
More info here.