Search_phase_execution_exception

hi all , i have 15 nodes in my Elasticsearch cluster, and each shards have 10GB , currently we have (1 shard+2 replicas ) ,we are using 6.18 elastic version , we are try ing to access data from api , but when i am running quires , all the time we are getting some error message {"message":"[search_phase_execution_exception] ","status":"error","statusCode":503,"meta":{"totalElapsedTime":"4.466 sec"}}, how to fix this error .
i am getting above message most of my quires .can any one help how to fix this error .

Is there more to the error?
What do the Elasticsearch logs show?

[INFO ][o.e.t.LoggingTaskListener] [node1] 73066301finished with response BulkByScrollResponse[took=3.7s,timed_out=false,sliceId=null,updated=59,created=0,deleted=0,batches=1,versionConflicts=0,noops=0,retries=0,throttledUntil=0s,bulk_failures=,search_failures=]