I cannot perform a search in mail

Check your internet connectivity and speed, and also sync status. Try restarting the device. Also, confirm the Fetch status in Settings -> Mail -> Accounts -> Fetch New Data -> Push is turned off or on, and email Fetching for accounts are set to Manual or Fetch. Also, Sync period. After checking all these, now hit the search in the Mail app with your search term. Above all make sure such a term exists, or you'll end up with related terms of search results, or No results found.

Below provided steps allow your phone to automatically download email from the server with the exact options chosen.

Open Settings -> Mail -> Accounts -> Fetch New Data -> Push(turned On), Fetch -> Automatically.

Now, make sure that the email you are searching for does exist.

Restart your device, and make sure you are connected to strong internet connectivity.

Now open the Mails app, and search for the email. If the keyword of the email matches with the search term provided, you'll see what you are searching for.

If no such mail exists, then no results found will be displayed.