update-database: "A network-related or instance-specific error occurred while establishing a connection to SQL Server"

I have a simple C# project. This is my connection string in my web.config for the database:

<connectionStrings>
   <add name="DefaultConnection" 
        connectionString="Data Source=172.17.0.47;Initial Catalog=DevSystemListe;User ID=web_access;Password=123456" 
        providerName="System.Data.SqlClient" />
</connectionStrings>

I already made sure that this connection is working. I can connect to my database from the Visual Studio with this connection and I can also see the tables and data.

When I want to update my database with update-database, this error occurs:

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

I uninstalled Entity Framework and then installed it again. Nothing changed.

I finally found out. I have a website project and my API in my solution. The problem was that the website was set as startup project. So that entity framework searched the connection string in the website's web.config and not in the api's web.config. Changing the startup project to my API solved the problem.

If you want to have the website project as startup project. Just copy the connection string into the websides web.config.

Nevertheless thanks for everyones help.

Also for migrations you can use OnConfiguring EntityFramework will check it by default. Add this code to your DbContext.

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder.UseSqlServer(@"connection_string_here");
}