Error when attempting to create a certificate request in Exchange Server 2013

Unable to generate CSR  – Please use a valid file name –  Exchange Server 2013

Please use a valid file name when you run the New-ExchangeCertificate cmdlet on Server on server with the -RequestFile parameter. The file should not exist in target folder. Parameter name: RequestFile

To ensure that you can download the certificate request to a particular network share, you must first assign the correct permissions to the file share you intend to use.

As illustrated below, grant the Exchange Trusted Subsystem group with modify or greater permissions against your chosen file share

ExchangeTrustedSubsystem