error when installing Exchange 2013 server

I am attempting to install a new Exchange 2013 server in our environment, (we already have 3 Exchange 2013 servers).
The server is an exchange 2012 r2 and all prerequisites were installed. I selected mail and CAS roles to be installed. I get the following error:

The following error was generated when "$error.Clear();
          if ( ($server -eq $null) -and ($RoleIsDatacenter -ne $true) )
          {
            Update-RmsSharedIdentity -ServerName $RoleNetBIOSName
          }
        " was run: "Microsoft.Exchange.Data.DataValidationException: Database is mandatory on UserMailbox.
   at Microsoft.Exchange.Data.Directory.ADDataSession.Save(ADObject instanceToSave, IEnumerable`1 properties, Boolean bypassValidation)
   at Microsoft.Exchange.Management.Deployment.UpdateRmsSharedIdentity.Link()
   at Microsoft.Exchange.Management.Deployment.UpdateRmsSharedIdentity.InternalProcessRecord()
   at Microsoft.Exchange.Configuration.Tasks.Task.<ProcessRecord>b__b()
   at Microsoft.Exchange.Configuration.Tasks.Task.InvokeRetryableFunc(String funcName, Action func, Boolean terminatePipelineIfFailed)".

Any assistance you can provide will be very much appreciated.

Thank you,