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From the error message, it seems that you are using DSS REST API .NET SDK. Please see the method to configure proxy in this question.
Cash flow bond structure using Datascope Select API
DSS and TRTH file Validating
Looking for Webservice link from Refinitiv
FuturesAndOptionsSearch - Results are not consistent
In DSS SOAP API : How to extract reference data (static data) for instruments and sample C# code to extract end of day prices for equities