There are a few causes for this error.
Please review each of the 'possible' causes below and their fixes to troubleshoot this error:
1. The incorrect alias was specified while trying to import the Certificate.
For further information on this, please refer to the following solution: SO8160
2. The Certificate being imported is improperly formatted.
For further information on this, please refer to the following solution: SO8162
3. The Certificate is imported into the incorrect keystore.
If the keystore is lost a new keystore will need to be generated, and the certificate reissued: SO5989