How to resolve SQL Exception 0x80131904?

I've connected a SQL Server to an ASP .Net Core app, but now, everytime I launch the app, or call "Update-Database", in Visual Studio, returns the error

Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.

Also, I'm trying to deploy the app to an IIS Site, and when I browse it, the Server Manager log shows a bunch of errors, which expection is always ...

An unhandled exception has occurred while executing the request.

Exception: Microsoft.Data.SqlClient.SqlException (0x80131904): Login failed for user ...

... indicating an user that has no access to the SQL Server.