The problem might be in your connection string.
"Server=localhost;Database=DatabaseNameHere;Trusted_Connection=True;MultipleActiveResultSets=true"
Ensure it uses either localhost or localhost\SQLEXPRESS, depending on your SQL Server installation.
The microsoft docs describe the exact steps to resolve this issue, it almost always has something to do with a typo in de name of the server instance.
https://blogs.msdn.microsoft.com/sql_protocols/2007/05/13/sql-network-interfaces-error-26-error-locating-serverinstance-specified/
To quote:
In a word, the reason that we get this error message is the client stack could not receive SSRP response UDP packet from SQL Browser. It's easy to isolate the issue. Here are the steps:
1) Make sure your server name is correct, e.g., no typo on the name.
2) Make sure your instance name is correct and there is actually such an instance on your target machine. [Update: Some application converts \ to . If you are not sure about your application, please try both ServerInstance and Server\Instance in your connection string]
3) Make sure the server machine is reachable, e.g, DNS can be resolve correctly, you are able to ping the server (not always true).
4) Make sure SQL Browser service is running on the server.
5) If firewall is enabled on the server, you need to put sqlbrowser.exe and/or UDP port 1434 into exception.
