Overuse of real-time search can result in performance costs and in this you are not able to scheduled a real-time alert because of restrictions that have applied in your environment
Options for restricting real-time search are as follows:
1) Disable real-time search at the indexer level by editing indexes.conf for specific indexes.
2) Disable real-time search for particular roles and users.
3) Edit limits.conf to reduce the number of real-time searches that can be run concurrently at any given time.
4) Edit limits.conf to restrict indexer support for real-time searches.
The documentation, How to restrict usage of real-time search is where you will want to go.
Also, make sure you're reading the documentation for your version of Splunk.
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