The error message explains it all. Searches with tstats can't be used for real-time searches. What is your requirement here?
Not only will it never work but it doesn't even make sense how it could. Use
stats instead and have it operate on the events as they come in to your real-time window. Better yet, do not use real-time! It almost certainly will not give you what you desire and it will crater the performance of your splunk cluster.