Upgrade to .NET Core 3.1 - Using StructureMap in .NET 3.1

In short, you should stop using StructureMap. It's no longer supported. Instead, use Lamar, and only have to make minimal changes. It's written by the same guy who wrote StructureMap.

In your Program.cs:

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .UseLamar()
                .ConfigureWebHostDefaults(webBuilder =>
                {

In your Startup.cs:

        public void ConfigureContainer(ServiceRegistry services)
        {
             ...
services.Scan(scan =>
             {
...