The search request was unable to connect to the Search Service.

Getting user preference failed: Microsoft.Office.Server.Search.Query.SearchServiceNotFoundException: The search request was unable to connect to the Search Service.

Cause : The search service application proxy is responsible for the communication between the search webpart and the search service application. Also the proxy application may be corrupted.

Resolution :  If you already have a search service application proxy, then delete it. Now run the below command to create a new proxy

$searchServApp = Get-SPEnterpriseSearchServiceApplication
New-SPEnterpriseSearchServiceApplicationProxy -SearchApplication $searchServApp