While using 'Set Data Source Credentials' task in Automation Services, the following error can be seen if the correct x.509 certificate is not being used.
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Running Set Data Source Credentials (1 of 1) 
The task 'Set Data Source Credentials' could not be executed. 
System.Security.Cryptography.CryptographicException: Unable to retrieve the decryption key. 
at System.Security.Cryptography.Xml.EncryptedXml.GetDecryptionKey(EncryptedData encryptedData, String symmetricAlgorithmUri) 
at System.Security.Cryptography.Xml.EncryptedXml.DecryptDocument() 
at Spotfire.Dxp.Automation.Tasks.Tasks.Implementation.X509Utilities.Decrypt(String encryptedData) 
at Spotfire.Dxp.Automation.Tasks.Tasks.SetInformationModelCredentials.ExecuteCore(TaskExecutionContext context) 
at Spotfire.Dxp.Automation.Extension.Task.Execute(TaskExecutionContext context) 
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Cause:
If the certificate was not created with a valid CSR, this error can be seen.

Error: The task 'Set Data Source Credentials' could not be executed

All supported environments

Make sure the certificate that is being used is the correct certificate. All the fields while generating the CSR must be correctly specified.

* Another reason behind the error could be that the local user(if the task fails locally) or the node manager service account(if the task fails on server) does not have access to this certificate.